Nationwide time trends in co-occurring mental illness and substance use disorders among adolescents and young adults: differences in diagnoses, age, gender, and educational attainment between 2010 and 2022

Nationwide time trends in co-occurring mental illness and substance use disorders among adolescents and young adults: differences in diagnoses, age, gender, and educational attainment between 2010 and 2022 | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Nationwide time trends in co-occurring mental illness and substance use disorders among adolescents and young adults: differences in diagnoses, age, gender, and educational attainment between 2010 and 2022 Josefine Skou Jakobsen, Katrine Schepelern Johansen, Birgitte Thylstrup, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7115166/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract This study aims to describe time trends in dual diagnosis occurrence among adolescents and young adults (ages 12–25) between 2010 and 2022 in Denmark. It also aims to describe how incident cases are distributed by age, gender, diagnoses, educational attainment, and enrollment in substance use treatment services. The study used diagnostic data on every admission to public mental health hospitals in Denmark linked with data from other national social service- and educational registers. Our analyses show a substantial increase in dual diagnosis occurrence in 2010-17, followed by a declining trend. When distributed by gender, age and diagnoses, dual diagnosis was most frequently occurring among males; young adults (ages 22–25); adolescents and young adults with schizophrenia, schizotypal and delusional disorders; neurotic, stress-related and somatoform disorders; as well as cannabis related disorders across the study period. Our analyses also show an increase in the proportion who were enrolled in substance use treatment at the time of their first dual diagnosis contact with mental health services, but a decreasing trend in the proportion who enrolled within a year after their first dual diagnosis contact. Finally, our analyses show a slight increase in the number of adolescents and young adults with dual diagnosis above 16 years of age who had completed primary school (9th grade) during the study period. Dual diagnosis mental illness substance use disorders time trends gender differences educational attainment Figures Figure 1 Figure 2 Introduction Accumulated evidence shows that mental health is deteriorating among adolescents and young adults worldwide [1]. Mental illness including substance use disorders (SUDs) is the leading cause of the global disease burden among adolescents and young adults [2, 3], and in European countries, this burden has increased over the past three decades [4]. People with mental illness are more likely to experience SUDs and vice versa [5], which makes co-occurrence of mental illnesses and SUDs (dual diagnosis) common [6-9]. Despite the common co-occurrence, mental illnesses and SUDs have for decades been treated in separate services in Denmark as well as in most European countries, resulting in inadequate and incoherent care for people with dual diagnosis [10-13]. Like mental illnesses in general, the onset of dual diagnosis often occurs in adolescence or young adulthood [1, 6], where important neurobiological, psychological, and social developmental processes take place [14-16]. Hence, adolescence and young adulthood are crucial phases of life for treatment and research efforts, with great potential for preventing exacerbation of mental illness symptoms and substance use as well as the many social and health-related problems associated with dual diagnosis. These include a range of interacting problems such as those related to physical health, housing, employment, education, economy, and incarceration, as well as increased risk of overdose, suicide, and overall premature mortality [17-21]. With increasing pressure on mental health systems across European countries to respond to the deteriorating mental health among adolescents and young adults, there is a great need to understand the most recent trends in dual diagnosis occurrence among this particularly vulnerable group to further qualify and tailor interventions to their needs. While it is well-established that there is a rise in admissions to child and adolescent mental health services across European countries [22-24], the literature on dual diagnosis occurrence in general is scarce [25], and even scarcer when it comes to time trends in different diagnoses and sociodemographic characteristics, including age, gender, and educational attainment. Furthermore, most studies are conducted in the US, resulting in limited evidence from the European context [11, 12]. The most recent study from Denmark found that the proportion of adolescents and young adults with dual diagnosis constituted about 20% of all adolescents and young adults treated in the mental health services in 2000, while this proportion rose to about 24% in 2017 [26]. However, like most other studies, this study did not differ between ages below and above 18, and little is known about whether trends in child and adolescent mental health services (ages18). Regarding gender differences, a recent systematic review found higher prevalence of dual diagnosis among young males than among young females [25]. Likewise, the aforementioned study on trends in dual diagnosis occurrence in Denmark found that the share of mental health patients with dual diagnosis were larger among males than females, and that the share rose only among males between 2000 and 2017 [26]. No studies exist that confirm whether a similar trend is seen among young males (ages 12-25) specifically. Regarding educational attainment, the association between mental illness and SUDs in adolescence and young adulthood and low educational attainment, school drop-out, and lower socioeconomic position later in life is well established [27-32]. Nonetheless, most studies report on the associations between educational attainment, mental illness, and SUDs separately, resulting in lack of knowledge on educational attainment among adolescents and young adults with dual diagnosis. One of the few studies reporting on educational attainment among this group shows that completing high school in a US context is associated with lower risks of dual diagnosis [33]. This underscores the preventive effect of education and highlights the crucial need to focus on educational attainment among adolescents and young adults with dual diagnosis in a Danish context. The study at hand aims to describe time trends in incidence and prevalence of dual diagnosis among adolescents and young adults aged 12-25 years who were treated in mental health services throughout Denmark between 2010 and 2022. To further characterize this group, this study also aims to describe how incident cases are distributed by age, gender, psychiatric and SUD diagnoses, and educational attainment prior to first dual diagnosis contact with the mental health services. Furthermore, this study aims to describe time trends in the proportion of adolescents and young adults with dual diagnosis who were enrolled in substance use treatment services during the same period. Methods Setting The total population in Denmark is 6 million people of which approximately 1 million are adolescents and young adults aged 12-25 years [34]. Coverage in the public national health care system, including the mental health services, is universal and primarily tax-funded with some co-payment. In the case of mental health treatment services for people under 25 years, co-payment is limited to some medications. While there has been an increase in the use of private mental health facilities in recent years, most patients are still treated in the public national mental health services [35, 36]. As with the health care system, the education system is also universal. Educational attainment equivalent to primary school (up to 9 th grade) is compulsory, and public-school enrollment is free of charge. Register data To identify young people with dual diagnosis, we retrieved data from the National Patient Register (NPR), which contains diagnostic data on every admission to public mental health hospitals in Denmark, including emergency, inpatient and outpatient care [37]. This register was also used to identify specific psychiatric and SUD diagnoses. To identify age and gender, we linked data from the NPR with the Danish Civil Registration System, which contains data on the unique personal identification number, gender, date of birth, and citizenship on all individuals living in Denmark [38]. To identify educational level and standardize the analyses by age and gender, we linked data from the NPR with national population-level data and educational registers available through Statistics Denmark, which is the central authority on societal statistics [39, 40]. To identify the number of adolescents and young adults with dual diagnosis who were also enrolled in substance use treatment services, we retrieved data from the National Alcoholism Treatment Register and the National Registry of Drug Abusers Undergoing Treatment , which contain data on every admission to public substance use treatment services, including dates of enrollment and discharge [41, 42]. Definition of variables Mental illness and SUD We defined mental illnesses and SUDs according to the WHO ICD-10 F chapter “Mental and behavioural disorders” (F00-F99). We identified one primary psychiatric diagnosis (excluding SUDs) for each person using the hierarchical structure of the ICD-10 chapter. We identified SUDs according to the section “Mental and behavioural disorders due to psychoactive substance use” (F10-F19). All substances were included except tobacco (F17), which is a seldomly used diagnostic category in the Danish mental health services. Dual diagnosis We defined dual diagnosis as the occurrence of a SUD and a mental illness within the same course of treatment by the mental health services. A course of treatment includes anything from a single contact with one mental health facility to admissions at different mental health facilities that followed on another and thus were part of the same treatment. Three ICD-10 subdivisions of SUDs were included in the definition of dual diagnosis: harmful use (F1x.1), dependence syndrome (F1x.2), and withdrawal state (F1x.3). Gender and age We included gender as a binary variable (M/F) based on the data available in the NPR, where gender is automatically registered based on a personal identification number. Age was grouped in three subgroups: 12-17, 18-21, and 22-25. Educational attainment We defined educational attainment as the highest attained educational level the year prior to a person’s first registered occurrence of dual diagnosis. We dichotomized educational attainment to analyze the percentage of adolescents and young adults who had not completed primary school, i.e., where the highest attained education was below the 9 th grade in the Danish education system. We therefore only included individuals aged 16 years and above, which is the typical age for completing 9 th grade in Denmark. Analysis For each year (2010-2022), we calculated the number of new dual diagnosis cases (incident cases), and the number of unique persons with dual diagnosis (prevalent cases). We standardized incident cases by age and gender using direct standardization with national population-level data on age and gender distribution in 2016 as reference. We further stratified incident cases for each year by age, gender, primary psychiatric diagnosis, SUD diagnosis, combinations of psychiatric and SUD diagnoses, and educational attainment. For each year, we also calculated the proportion of adolescents and young adults with dual diagnosis (incident cases) who were undergoing treatment in the substance use services. We did this in two ways: 1) the proportion of adolescents and young adults with dual diagnosis who were already enrolled in substance use treatment services by the time of their first dual diagnosis contact with the mental health services, and 2) the proportion enrolled in substance use treatment services within a year after their first dual diagnosis contact with the mental health services. Data was analyzed using SAS version 9.4. Ethical approval The project was approved by the Regional Data Protection Agency (no. P-2022-616). No further ethical approval is required by Danish Law for studies that are entirely based on register data. Results Figure 1a shows that the number of prevalent cases of dual diagnosis among adolescents and young adults has increased remarkably during the study period. The number of prevalent cases increased by 57% between 2010 (1856 cases) and 2020 (2919 cases), followed by a slight declining trend between 2020 and 2022. It also shows that the number of incident cases rose significantly between 2010 and 2017, from 1176 to 1563 new cases. From 2017 and onwards, however, there is a declining trend in the number of new cases, excluding the peak in 2019 which should be interpreted with caution due to data breach in the NPR between 2018 and 2019 [43]. Figure 1b shows that the number of incident and prevalent cases follows a similar trend over the study period when standardized by age and gender, with standardized rates peaking in 2017 followed by a declining trend. Between 2017 and 2022, the prevalence rate declined from 30.4 to 26.2/10,000 and the incidence rate declined from 15.3 to 9.7/10,000 adolescents and young adults in dual diagnosis treatment. Figure 1c shows that incident cases are highest among the 22–25-year-olds and lowest among the 12-17-year-olds during the entire period. However, there is a difference in trends among the three age groups between 2010 and 2017, where we see a distinct increase in incident cases among the 12-17-year-olds compared to the two other age groups. Mean age for incident cases is around 20 during the entire study period. Figure 1d shows that the number of incident cases is remarkably higher among males than females during the entire period, with the number of new cases ranging between 561 in 2022 as the lowest and 1011 in 2015 as the highest among males, and between 396 in 2018 as the lowest and 577 in 2017 as the highest among females. Figure 1e shows that schizophrenia, schizotypal, and delusional disorders (F20-29) as well as neurotic, stress-related, and somatoform disorders (F40-49) contribute most to the number of new cases during the entire study period. The number of new cases remains relatively consistent across diagnostic groups, except for neurotic, stress-related, and somatoform disorders (F40-49), where we see a distinct increase in incident cases between 2010 and 2017. However, this increase is followed by a decline between 2017 and 2022. From 2017, there is also a remarkable decline in incident cases of disorders of adult personality and behaviour (F60-69) and schizophrenia, schizotypal and delusional disorders (F20-29). Figure 1f shows that cannabis related disorders (F12) are by far the most common SUD during the entire study period, as they are present in around 50-60% of new cases of dual diagnosis. It also shows that there is a rise in cocaine related disorders and disorders related to other stimulants (F14-15), from 7% of new cases in 2010 to 13% of new cases in 2022. We also see a slight increase in alcohol related disorders (F10) with a peak in 2022, where they are present in around 24% of new cases of dual diagnosis. Figure 1g shows that across the period, 12% to 18% of adolescents and young adults were already enrolled in substance use treatment at their first dual diagnosis contact with the mental health services, with an increasing trend towards the end of the study period. Of those who were not already enrolled in substance use treatment, 17% to 24% enrolled in substance use treatment within a year after their first dual diagnosis contact with the mental health services. Here, we see an increasing trend between 2010 and 2020, followed by a steep decline, with the lowest numbers in 2022. Figure 1h shows that there has been a slight increase in the number of adolescents and young adults above the age of 16 with dual diagnosis who have completed primary school. Nonetheless, despite the increasing trend, the number of adolescents and young adults with dual diagnosis who have not completed primary school remains stable at 7-8% from 2017 and onwards. Figure 2a shows a distinct increase in cannabis related disorders (F12) among adolescents and young adults with schizophrenia, schizotypal and delusional disorders (F20-29), while figure 2c shows a slight increase among those with neurotic, stress-related and somatoform disorders (F40-49). On the contrary, figure 2b and 2d show that cannabis related disorders (F12) are declining among adolescents and young adults with mood (affective) disorders (F30-F39), and behavioural and emotional disorders with onset usually occurring in childhood and adolescence (F90-F98). Figure 2b and 2d also show a distinct increase in alcohol related disorders (F10) among adolescents and young adults with mood (affective) disorders (F30-F39), and behavioural and emotional disorders with onset usually occurring in childhood and adolescence (F90-F98). For all four groups of psychiatric diagnoses (figures 2a-d), we see a rise in disorders related to cocaine and other stimulants (F14-15) across the study period. Discussion In this study, we found that while the incidence and prevalence of dual diagnosis among adolescents and young adults increased substantially during the period 2010-2017, there was a declining trend from 2017 to 2022, for incident cases in particular. When distributed by age and gender, the absolute number of new cases remained highest among the oldest age group (22-25 years) and lowest among the youngest age group (12-17 years), and higher among males compared to females during the study period. Further, when incident cases were distributed by the combination of psychiatric diagnoses and SUDs, we found: a distinct increase in cannabis related disorders (F12) among adolescents and young adults with schizophrenia, schizotypal and delusional disorders (F20-29); a distinct increase in alcohol related disorders (F10) among adolescents and young adults with mood (affective) disorders (F30-F39), and behavioural and emotional disorders with onset usually occurring in childhood and adolescence (F90-F98); and a rise in disorders related to cocaine and other stimulants (F14-15) across the four most frequently occurring psychiatric diagnoses. We found an increase in the proportion of adolescents and young adults with dual diagnosis who were enrolled in substance use treatment at the time of their first dual diagnosis contact with the mental health services. Among those who were not yet enrolled in substance use treatment, we found an increase followed by a decrease towards the end of the period in the proportion who enrolled in substance use treatment within a year after their first dual diagnosis contact with the mental health services. Finally, we found a slight increase in the number of adolescents and young adults with dual diagnosis above 16 years of age who had completed primary school (9 th grade) during the period. However, the proportion who had not completed primary school remained at persistently concerning levels. Comparison with other studies Potential underdetection of dual diagnosis among adolescents and young adults In line with global tendencies [1], an increasing number of adolescents and young adults in Denmark are diagnosed with a mental illness and self-report poor mental health [22, 44]. A recent study from Denmark found increasing rates of mental illness among young people from the first lockdown during the Covid-19 pandemic in 2020 to 2022 [45]. Based on this we would have expected to find a substantial increase in the number of new cases of dual diagnosis during the same period in this study, both regarding overall numbers and, in particular, among the youngest age groups (ages 12-17). On the contrary, we found that the number of new cases increased between 2010 and 2017 followed by a declining trend, and that the number of new cases remained lower in the youngest age group compared to the older age groups. The decline after 2019 could be explained by a general decline in substance use among youth in Denmark during the Covid-19 pandemic [46]. Recent national data also shows a decline in young people registered in drug treatment services in Denmark between 2016-2022 [47]. On the contrary, in the same period, there was an increase in the number of young people registered in alcohol treatment services [48]. The declining trend from 2017 and onwards could also be related to the general increase in mental illness among adolescents and young adults in Denmark: with increasing pressure on the mental health system, it is likely that SUDs may not have been systematically screened for, diagnosed and reported, hence contributing to the decline in dual diagnosis occurrence. Persistent underdiagnosis of dual conditions has been reported in other studies [26], which may also help explain the trend across the entire study period: from 1176 new cases in 2010 to 1007 in 2021. Another possible explanation for the declining trend relates to the extensive implementation of new electronic health record- and register systems in the Danish mental health services in 2016-2019, which may have affected reporting practices and hence the diagnostic data [49]. Despite the decreasing trend in the number of new cases, we found a general rise in prevalent cases during the study period, with a far less distinct decrease towards the end of the period compared with incident cases. This indicates a tendency toward longer treatment durations, which may be interpreted in two ways: that mental health services are increasingly more successful in maintaining adolescents and young adults in treatment, or, more concerningly, a tendency of increasing readmissions to mental health services. Consistent gender differences in dual diagnosis The international literature points toward a ubiquitous cross-cultural gender gap in youth mental health, with girls and young women having worse average mental health than boys and young men [50, 51]. However, young men are disproportionally affected when it comes to SUDs [2], which may explain our findings of consistent gender differences in dual diagnosis incidence across the entire study period. A recent report on substance use among women across European countries finds that women more often than men are stigmatized on the basis of their substance use, which, in combination with discriminatory and gender-blind services, may deter help-seeking [52]. The same report also highlights that women with dual diagnosis are more likely to be reported as having a SUD subsequent to a primary mental illness, indicating that exclusion from substance use services may occur more frequently among women than men [52]. Another recent study points to similar barriers to accessing care among women with dual diagnosis, and advocates for gender-based perspectives to be applied in studies and treatment of dual diagnosis [53]. While these findings are not specific to adolescents and young adults, they point toward a potential under-detection of dual diagnosis among girls and young women, which may contribute to explain the consistent gender differences in dual diagnosis incidence found in this study. Persistently concerning levels of educational disengagement The Danish welfare system is characterized by universal and free access to education, including compulsory engagement in primary school up to 9 th grade. Nevertheless, our findings indicate persistent challenges with educational engagement among adolescents and young adults with dual diagnosis compared with the general population: a recent study with data from 2021 shows that only 0.5% of all young adults in Denmark (ages 18-24) had not completed primary school [54]. Other recent studies with data from Denmark and Norway found an educational gradient in years lived with disability from mental illness and SUDs [55] as well as an association between self-reporting low social status in school and lack of support from teachers or classmates and later depressive symptom trajectories [56]. International evidence confirms and nuances this persistent challenge: a systematic review on school drop-out found a strong association between early life externalizing disorders (including SUDs) and lower educational attainment, while internalizing disorders on the other hand were reported as a consequence of school drop-out [57]. Considering this evidence, our findings indicate that not finishing primary school may be an early sign of poor mental health which has not yet been detected. This underscores the need for early interventions to prevent the exacerbation of mental illness symptoms and substance use as well as the socioeconomic consequences associated with not finishing primary school. The global drug market and trends in substance use In line with global tendencies and recent findings from Denmark showing an increase in use of non-prescribed opioids among adolescents and young adults [58], we would have expected a rise in new cases of dual diagnosis related to opioid use. Instead, we found a stable trend with a continuously low proportion of new cases. While both cannabis related disorders (F12) and schizophrenia, schizotypal, and delusional disorders (F20-29) seem to be declining among adolescents and young adults with dual diagnosis towards the end of the study period, we found a notably distinct rise in cannabis related disorders (F12), when looking only at new cases of dual diagnosis related to schizophrenia, schizotypal and delusional disorders (F20-29). This rise may be interpreted as caused by the increased potency of cannabis, as shown in recent studies from Denmark [59, 60]. Similarly, our findings showing an increase in new cases of dual diagnoses related to cocaine and other stimulants (F14-15) may be explained by the normalization of cocaine use among young people in European countries, which is emphasized in recent literature [61]. Strengths and limitations In our identification of adolescents and young adults with dual diagnosis, we only included those who had a SUD and a mental illness within the same course of treatment in the public mental health services. Furthermore, only three ICD-10 subdivisions of SUDs were deemed relevant in the definition of dual diagnosis: harmful use (F1x.1), dependence syndrome (F1x.2), and withdrawal state (F1x.3). While this narrow definition ensures certain reporting of the development in the number of adolescents and young adults with dual diagnosis who are treated in the mental health services, it comes at the expense of potentially underreporting dual diagnosis occurrence. Only using data from the NPR also contributes to a possible distorted view of dual diagnosis occurrence among adolescents and young adults in the general population, as demonstrated by Mårtensson et al [26], because the NPR only includes data on those who seek help in the mental health services. Relatedly, we linked data based on personal identification numbers, and consequently, adolescents and young adults without such number were not included in the analysis, i.e., undocumented migrants treated in emergency wards. However, the use of data exclusively from the comprehensive NPR is also the main strength of this study, as it covers all mental health services on a national scale. As mentioned, however, the data breach in the NPR between 2018 and 2019 is a limitation to our analysis [49]. Another limitation relates to the calculated decline in incident cases between 2021 and 2022, which may be less distinct than reported here, due to possibly incomplete data from 2022, where treatment courses that were still ongoing at the time of data retrieval were not included in the data set. Finally, it is a limitation to our analysis that we were only able to report on binary (M/F) gender differences, despite recent research shedding light on the disproportionate mental health and substance use problems among transgender, non-binary, and gender diverse youth [62, 63]. Research implications and clinical relevance While there is strong international evidence showing a ubiquitous decline in mental health including a rise in SUDs among adolescents and young adults, few studies report on the co-occurrence of mental illness and SUDs, not least on the time trends in such co-occurrences. Our study thus significantly contributes to the understanding of recent trends in dual diagnosis occurrence among adolescents and young adults. We find a declining trend in incident cases towards the end of the study period (2017-2022), which, as positive as this trend may seem, could be the result of persistent underdiagnosis. We therefore suggest further research into time trends in dual diagnosis occurrence among adolescents and young adults as well as into the factors that may influence the potential underdetection among this particularly vulnerable group. Integration of substance use services and mental health services is widely recognized as a means to ensure the most efficient treatment of dual diagnosis [64, 65]. In Denmark, integrated treatment is now gradually introduced in the mental health services, and it is expected to be fully implemented on a national scale in 2027 [66]. The success of such large-scale mental health intervention relies on – among other things – sufficient and updated knowledge about the people it intends to help. Hence, our findings on recent trends in dual diagnosis among adolescents and young adults provide a timely contribution to further qualify integrated treatment efforts, and tailor treatment to the needs of adolescents and young adults with dual diagnosis. Declarations Ethical approval The project was approved by the Regional Data Protection Agency (no. P-2022-616). No further ethical approval is required by Danish Law for studies that are entirely based on register data. Funding This study was funded by the Novo Nordisk Foundation under grant no. NNF22OC0077756. The Novo Nordisk Foundation is an independent, Danish enterprise foundation that supports scientific, humanitarian and social causes. The Foundation has neither been involved nor consulted in any steps of the research process. Competing interests The authors have no competing interests to declare that are relevant to the content of this article. Author Contribution All authors contributed to the study conception and design. JSJ conducted the data analysis and interpretation. SM revised the data analysis and interpretation. JSJ wrote the main manuscript text, which was reviewed by all authors. 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Et stigende antal speciallæger har hovedbeskæftigelse i det private 2023 [Available from: https://ugeskriftet.dk/nyhed/et-stigende-antal-speciallaeger-har-hovedbeskaeftigelse-i-det-private. Plana-Ripoll O, Liu X, Köhler-Forsberg O, Sørensen HT, Momen NC. Mental Disorders in Danish Hospital Registers: A Review of Content and Possibilities for Epidemiological Research. Clin Epidemiol. 2025;17:387-407. https://doi.org/10.2147/clep.S509147 Pedersen CB. The Danish Civil Registration System. Scandinavian Journal of Public Health. 2011;39(7):22-5. https://doi.org/10.1177/1403494810387965 Jensen VM, Rasmussen AW. Danish Education Registers. Scand J Public Health. 2011;39(7 Suppl):91-4. https://doi.org/10.1177/1403494810394715 Statistics Denmark. [Available from: https://www.dst.dk/en. Schwarz AS, Nielsen B, Nielsen AS. Changes in profile of patients seeking alcohol treatment and treatment outcomes following policy changes. Z Gesundh Wiss. 2018;26(1):59-67. https://doi.org/10.1007/s10389-017-0841-0 Pedersen MU, Hesse M, Thylstrup B. Research in substance abuse treatment: Contributions by the Centre for Alcohol and Drug Research. Drugs: Education, Prevention and Policy. 2013;20(6):435-42. https://doi.org/10.3109/09687637.2013.840460 Statistics Denmark. Documentation of statistics for hospitalization 2023 [Available from: https://www.dst.dk/en/Statistik/dokumentation/documentationofstatistics/hospitalization/comparability. Jeppesen P, Obel C, Lund L, Madsen KB, Nielsen L, Nordentoft M. Mental sundhed og sygdom hos børn og unge i alderen 10-24 år - forekomst, udvikling og forebyggelsesmuligheder. Vidensråd for Forebyggelse; 2020. Bliddal M, Rasmussen L, Andersen JH, Jensen PB, Pottegård A, Munk-Olsen T, et al. Psychotropic Medication Use and Psychiatric Disorders During the COVID-19 Pandemic Among Danish Children, Adolescents, and Young Adults. JAMA Psychiatry. 2023;80(2):176-80. https://doi.org/10.1001/jamapsychiatry.2022.4165 Danish Health Authority. Udbredelse af illegale stoffer i befolkningen og blandt de unge. Narkotikasituationen i Danmark - delrapport 1. 2022, Danish Health Authority. Stofmisbrugsbehandling – efterspørgsel og til gængelighed: Narkotikasituationen i Danmark - Delrapport 3. 2024, Danish Health Data Authority. Still more people undergoing alcohol treatment 2023 [Available from: https://sundhedsdatastyrelsen.dk/nyheder/2023/stadig-flere-i-behandling-for-alkoholmisbrug. Statistics Denmark. Documentation of statistics for hospitalization 2022. [Available from: https://www.dst.dk/en/Statistik/dokumentation/documentationofstatistics/hospitalization/comparability. Campbell OLK, Bann D, Patalay P. The gender gap in adolescent mental health: A cross-national investigation of 566,829 adolescents across 73 countries. SSM - Population Health. 2021;13:100742. https://doi.org/10.1016/j.ssmph.2021.100742 World Health Organization, UNICEF. Child and adolescent health in the WHO European Region: adolescent mental health fact sheet. 2024. European Monitoring Centre for Drugs and Drug Addiction. Women and drugs: health and social responses. 2022. Torrens-Melich M, Orengo T, Rodríguez de Fonseca F, Almodóvar I, Baquero A, Benito A. Gender Perspective in Dual Diagnosis. Brain Sciences [Internet]. 2021; 11(8). Available from: https://doi.org/10.3390/brainsci11081101. Benjaminsen L, Enemark MH. Brug af velfærdssystemet blandt unge med dobbeltdiagnose – En registerbaseret undersøgelse af brug af indsatser og ydelser blandt unge med både en psykisk lidelse og et misbrug. VIVE; 2024. Weye NO, Plana-Ripoll O, Baravelli CM, Agardh EE, van der Velde L, Kinge JM, et al. Educational differences in years lived with disability due to mental and substance use disorders: a cohort study using nationwide Norwegian and Danish registries. BMC Public Health. 2024;24(1):2576. https://doi.org/10.1186/s12889-024-20064-0 Sørensen CLB, Plana-Ripoll O, Bültmann U, Winding TN, Steen PB, Biering K. Developmental trajectories in mental health through adolescence and adulthood: does socio-economic status matter? Epidemiology and Psychiatric Sciences. 2025;34:e33. https://doi.org/10.1017/S2045796025100073 Esch P, Bocquet V, Pull C, Couffignal S, Lehnert T, Graas M, et al. The downward spiral of mental disorders and educational attainment: a systematic review on early school leaving. BMC Psychiatry. 2014;14(1):237. https://doi.org/10.1186/s12888-014-0237-4 Esben Houborg, Maj Nygaard-Christensen, Birgitte Thylstrup. Unge og opioider - Danske unges veje ind i og ud af brug og afhængighed. Aarhus University; 2024. Hjorthøj C, Posselt CM, Nordentoft M. Development Over Time of the Population-Attributable Risk Fraction for Cannabis Use Disorder in Schizophrenia in Denmark. JAMA Psychiatry. 2021;78(9):1013-9. https://doi.org/10.1001/jamapsychiatry.2021.1471 Rømer Thomsen K, Lindholst C, Thylstrup B, Kvamme S, Reitzel LA, Worm-Leonhard M, et al. Changes in the composition of cannabis from 2000–2017 in Denmark: Analysis of confiscated samples of cannabis resin. Experimental and Clinical Psychopharmacology. 2019;27(4):402-11. https://doi.org/10.1037/pha0000303 Hellman M. Snow season: The normalisation of cocaine. Nordic Studies on Alcohol and Drugs. 2024;41(2):139-41. https://doi.org/10.1177/14550725241234732 Newcomb ME, Hill R, Buehler K, Ryan DT, Whitton SW, Mustanski B. High Burden of Mental Health Problems, Substance Use, Violence, and Related Psychosocial Factors in Transgender, Non-Binary, and Gender Diverse Youth and Young Adults. Archives of Sexual Behavior. 2020;49(2):645-59. https://doi.org/10.1007/s10508-019-01533-9 Plessen KJ, Kelly-Irving M. Sex-and gender-specific aspects in Child and Adolescent Psychiatry – a blind spot requiring our attention. European Child & Adolescent Psychiatry. 2025;34(6):1687-9. https://doi.org/10.1007/s00787-025-02749-y Brewer S, Godley MD, Hulvershorn LA. Treating Mental Health and Substance Use Disorders in Adolescents: What Is on the Menu? Current Psychiatry Reports. 2017;19(1):5. https://doi.org/10.1007/s11920-017-0755-0 Spencer AE, Valentine SE, Sikov J, Yule AM, Hsu H, Hallett E, et al. Principles of Care for Young Adults With Co-Occurring Psychiatric and Substance Use Disorders. Pediatrics. 2021;147(Suppl 2):229-39. https://doi.org/10.1542/peds.2020-023523F Ministry of the Interior and Health of Denmark. Model for et samlet tilbud for mennesker med brug for behandling af både en psykisk lidelse og et misbrug (dobbeltdiagnosebehandling) og for omlægning af de særlige pladser i psykiatrien. 2021. https://www.ft.dk/samling/20211/almdel/suu/spm/240/svar/1846580/2513707.pdf Additional Declarations No competing interests reported. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-7115166","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":503298314,"identity":"d76390d6-dc1b-4eca-949a-413678fce806","order_by":0,"name":"Josefine Skou Jakobsen","email":"data:image/png;base64,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","orcid":"","institution":"Mental Health Centre Copenhagen","correspondingAuthor":true,"prefix":"","firstName":"Josefine","middleName":"Skou","lastName":"Jakobsen","suffix":""},{"id":503298315,"identity":"5082d041-ddc7-46e7-86a1-99d21b2c036b","order_by":1,"name":"Katrine Schepelern Johansen","email":"","orcid":"","institution":"Copenhagen University Hospital Hvidovre","correspondingAuthor":false,"prefix":"","firstName":"Katrine","middleName":"Schepelern","lastName":"Johansen","suffix":""},{"id":503298316,"identity":"414f7c13-9aa3-45b0-8e27-9131c375d971","order_by":2,"name":"Birgitte Thylstrup","email":"","orcid":"","institution":"Aarhus University","correspondingAuthor":false,"prefix":"","firstName":"Birgitte","middleName":"","lastName":"Thylstrup","suffix":""},{"id":503298317,"identity":"4ce204f6-e7f0-49e4-9090-527b4447d396","order_by":3,"name":"Solvej Mårtensson","email":"","orcid":"","institution":"Mental Health Centre Copenhagen","correspondingAuthor":false,"prefix":"","firstName":"Solvej","middleName":"","lastName":"Mårtensson","suffix":""}],"badges":[],"createdAt":"2025-07-13 19:23:14","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-7115166/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-7115166/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":89974722,"identity":"56fa6482-e17c-4453-9646-d7d7ea08347f","added_by":"auto","created_at":"2025-08-27 05:55:00","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":155190,"visible":true,"origin":"","legend":"\u003cp\u003ePrevalent and incident cases of dual diagnosis among adolescents and young adults distributed by diagnoses, sociodemographic characteristics and enrollment in substance use treatment before/within a year after first dual diagnosis contact with the mental heatlh services.\u003c/p\u003e\n\u003cp\u003e* \u003cstrong\u003eF0-09\u003c/strong\u003e(Organic, including symptomatic, mental disorders); \u003cstrong\u003eF20-29 \u003c/strong\u003e(Schizophrenia, schizotypal and delusional disorders); \u003cstrong\u003eF30-39\u003c/strong\u003e(Mood (affective) disorders); \u003cstrong\u003eF40-49\u003c/strong\u003e(Neurotic, stress-related and somatoform disorders); \u003cstrong\u003eF50-59\u003c/strong\u003e(Behavioural syndromes associated with physiological disturbances and physical factors); \u003cstrong\u003eF60-69\u003c/strong\u003e (Disorders of adult personality and behaviour); \u003cstrong\u003eF70-79\u003c/strong\u003e (Mental retardation); \u003cstrong\u003eF80-89\u003c/strong\u003e (Disorders of psychological development); \u003cstrong\u003eF90-98\u003c/strong\u003e (Behavioural and emotional disorders with onset usually occurring in childhood and adolescence)\u003cstrong\u003e.\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e** F10 \u003c/strong\u003e(Mental and behavioural disorders due to use of alcohol);\u003cstrong\u003e F11 \u003c/strong\u003e(Mental and behavioural disorders due to use of opioids); \u003cstrong\u003eF12 \u003c/strong\u003e(Mental and behavioural disorders due to use of cannabinoids);\u003cstrong\u003e F13\u003c/strong\u003e (Mental and behavioural disorders due to use of sedatives or hypnotics); \u003cstrong\u003eF14-15 \u003c/strong\u003e(Mental and behavioural disorders due to use of cocaine \u0026amp;\u003cem\u003e \u003c/em\u003eMental and behavioural disorders due to use of other stimulants; \u003cstrong\u003eF19 \u003c/strong\u003e(Mental and behavioural disorders due to multiple drug use and use of other psychoactive substances).\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-7115166/v1/00ccd5fa794604e14376bf1b.png"},{"id":89976372,"identity":"d63ad13e-f776-40fd-8cb9-c8fe5064ffe7","added_by":"auto","created_at":"2025-08-27 06:03:00","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":101931,"visible":true,"origin":"","legend":"\u003cp\u003eIncident cases of the four most frequently occuring psychiatric diagnoses among adolescents and young adults with dual diagnosis distributed by SUDs.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003e* F10 \u003c/strong\u003e(Mental and behavioural disorders due to use of alcohol);\u003cstrong\u003e F11 \u003c/strong\u003e(Mental and behavioural disorders due to use of opioids); \u003cstrong\u003eF12 \u003c/strong\u003e(Mental and behavioural disorders due to use of cannabinoids);\u003cstrong\u003e F13\u003c/strong\u003e (Mental and behavioural disorders due to use of sedatives or hypnotics); \u003cstrong\u003eF14-15 \u003c/strong\u003e(Mental and behavioural disorders due to use of cocaine \u0026amp;\u003cem\u003e \u003c/em\u003eMental and behavioural disorders due to use of other stimulants; \u003cstrong\u003eF19 \u003c/strong\u003e(Mental and behavioural disorders due to multiple drug use and use of other psychoactive substances).\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-7115166/v1/9576a559858b7ab2ef109ef8.png"},{"id":91373735,"identity":"ea4a701f-d5dd-4473-b756-48bb38fa401f","added_by":"auto","created_at":"2025-09-15 19:23:28","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":662431,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7115166/v1/1b47dc60-543d-4845-be04-16d3795d5da1.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Nationwide time trends in co-occurring mental illness and substance use disorders among adolescents and young adults: differences in diagnoses, age, gender, and educational attainment between 2010 and 2022","fulltext":[{"header":"Introduction","content":"\u003cp\u003eAccumulated evidence shows that mental health is deteriorating among adolescents and young adults worldwide\u0026nbsp;[1]. Mental illness including substance use disorders (SUDs) is the leading cause of the\u0026nbsp;global disease burden among adolescents and young adults [2, 3], and in European countries, this burden has increased over the past three decades [4]. People with mental illness are more likely to experience SUDs and vice versa [5], which makes co-occurrence of mental illnesses and SUDs (dual diagnosis) common [6-9]. Despite the common co-occurrence, mental illnesses and SUDs have for decades been treated in separate services in Denmark as well as in most European countries, resulting in inadequate and incoherent care for people with dual diagnosis [10-13]. Like mental illnesses in general, the onset of dual diagnosis often occurs in adolescence or young adulthood [1, 6], where important neurobiological, psychological, and social developmental processes take place [14-16]. Hence, adolescence and young adulthood are crucial phases of life for treatment and research efforts, with great potential for preventing exacerbation of mental illness symptoms and substance use as well as the many social and health-related problems associated with dual diagnosis. These include a range of interacting problems such as those related to physical health,\u0026nbsp;housing, employment, education, economy, and incarceration, as well as increased risk of overdose, suicide, and overall premature mortality [17-21].\u003c/p\u003e\n\u003cp\u003eWith increasing pressure on mental health systems across European countries to respond to the deteriorating mental health among adolescents and young adults, there is a great need to understand the most recent trends in dual diagnosis occurrence among this particularly vulnerable group to further qualify and tailor interventions to their needs. While it is well-established that there is a rise in admissions to child and adolescent mental health services across European countries [22-24], the literature on dual diagnosis occurrence in general is scarce\u0026nbsp;[25], and even scarcer when it comes to time trends in different diagnoses and sociodemographic characteristics, including age, gender, and educational attainment.\u0026nbsp;Furthermore, most studies are conducted in the US, resulting in limited evidence from the European context\u0026nbsp;[11, 12].\u0026nbsp;The most recent study from Denmark found that the proportion of adolescents and young adults with dual diagnosis constituted about 20% of all adolescents and young adults treated in the mental health services in 2000, while this proportion rose to about 24% in 2017\u0026nbsp;[26]. However, like most other studies, this study did not differ between ages below and above 18, and little is known about whether trends in child and adolescent mental health services (ages\u0026lt;18) differ from those in adult mental health services (ages\u0026gt;18).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eRegarding gender differences, a recent systematic review found higher prevalence of dual diagnosis among young males than among young females [25]. Likewise, the aforementioned study on trends in dual diagnosis occurrence in Denmark found that the share of mental health patients with dual diagnosis were larger among males than females, and that the share rose only among males between 2000 and 2017 [26]. No studies exist that confirm whether a similar trend is seen among young males (ages 12-25) specifically.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eRegarding educational attainment, the association between mental illness and SUDs in adolescence and young adulthood and low educational attainment, school drop-out, and lower socioeconomic position later in life is well established [27-32]. Nonetheless, most studies report on the associations between educational attainment, mental illness, and SUDs separately, resulting in lack of knowledge on educational attainment among adolescents and young adults with dual diagnosis. One of the few studies reporting on educational attainment among this group shows that completing high school in a US context is associated with lower risks of dual diagnosis [33]. This underscores the preventive effect of education and highlights the crucial need to focus on educational attainment among adolescents and young adults with dual diagnosis in a Danish context.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe study at hand aims to describe time trends in incidence and prevalence of dual diagnosis among adolescents and young adults aged 12-25 years who were treated in mental health services throughout Denmark between 2010 and 2022. To further characterize this group, this study also aims to describe how incident cases are distributed by age, gender, psychiatric and SUD diagnoses, and educational attainment prior to first dual diagnosis contact with the mental health services. Furthermore, this study aims to describe time trends in the proportion of adolescents and young adults with dual diagnosis who were enrolled in substance use treatment services during the same period. \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eSetting\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe total population in Denmark is 6 million people of which approximately 1 million are adolescents and young adults aged 12-25 years [34]. Coverage in the public national health care system, including the mental health services, is universal and primarily tax-funded with some co-payment. In the case of mental health treatment services for people under 25 years, co-payment is limited to some medications. While there has been an increase in the use of private mental health facilities in recent years, most patients are still treated in the public national mental health services [35, 36]. As with the health care system, the education system is also universal. Educational attainment equivalent to primary school (up to 9\u003csup\u003eth\u003c/sup\u003e grade) is compulsory, and public-school enrollment is free of charge.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eRegister data\u003c/p\u003e\n\u003cp\u003eTo identify young people with dual diagnosis, we retrieved data from the National Patient Register (NPR), which contains diagnostic data on every admission to public mental health hospitals in Denmark, including emergency, inpatient and outpatient care [37]. This register was also used to identify specific psychiatric and SUD diagnoses. To identify age and gender, we linked data from the NPR with the\u003cem\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003eDanish Civil Registration System, which contains\u0026nbsp;\u003c/em\u003edata on the unique personal identification number, gender, date of birth, and citizenship on all individuals living in Denmark\u003cem\u003e\u0026nbsp;\u003c/em\u003e[38].\u0026nbsp;To identify educational level and standardize the analyses by age and gender, we linked data from the NPR with national population-level data and educational registers available through Statistics Denmark, which is the central authority on societal statistics\u0026nbsp;[39, 40]. To identify the number of adolescents and young adults with dual diagnosis who were also enrolled in substance use treatment services, we retrieved data from the National Alcoholism Treatment Register and \u003cem\u003ethe National Registry of Drug Abusers Undergoing Treatment\u003c/em\u003e, which contain data on every admission to public substance use treatment services, including dates of enrollment and discharge\u0026nbsp;[41, 42]. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eDefinition of variables\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eMental illness and SUD\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eWe defined mental illnesses and SUDs according to the WHO ICD-10 F chapter \u0026ldquo;Mental and behavioural disorders\u0026rdquo; (F00-F99). We identified one primary psychiatric diagnosis (excluding SUDs) for each person using the hierarchical structure of the ICD-10 chapter. We identified SUDs according to the section \u0026ldquo;Mental and behavioural disorders due to psychoactive substance use\u0026rdquo; (F10-F19). All substances were included except tobacco (F17), which is a seldomly used diagnostic category in the Danish mental health services.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eDual diagnosis\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eWe defined dual diagnosis as the occurrence of a SUD and a mental illness within the same course of treatment by the mental health services. A course of treatment includes anything from a single contact with one mental health facility to admissions at different mental health facilities that followed on another and thus were part of the same treatment. Three ICD-10 subdivisions of SUDs were included in the definition of dual diagnosis: harmful use (F1x.1), dependence syndrome (F1x.2), and withdrawal state (F1x.3).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eGender and age\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eWe included gender as a binary variable (M/F) based on the data available in the NPR, where gender is automatically registered based on a personal identification number. Age was grouped in three subgroups: 12-17, 18-21, and 22-25.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eEducational attainment\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eWe defined educational attainment as the highest attained educational level the year prior to a person\u0026rsquo;s first registered occurrence of dual diagnosis. We dichotomized educational attainment to analyze the percentage of adolescents and young adults who had not completed primary school, i.e., where the highest attained education was below the 9\u003csup\u003eth\u003c/sup\u003e grade in the Danish education system. We therefore only included individuals aged 16 years and above, which is the typical age for completing 9\u003csup\u003eth\u003c/sup\u003e grade in Denmark. \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAnalysis\u003c/p\u003e\n\u003cp\u003eFor each year (2010-2022), we calculated the number of new dual diagnosis cases (incident cases), and the number of unique persons with dual diagnosis (prevalent cases). We standardized incident cases by age and gender using direct standardization with national population-level data on age and gender distribution in 2016 as reference. We further stratified incident cases for each year by age, gender, primary psychiatric diagnosis, SUD diagnosis, combinations of psychiatric and SUD diagnoses, and educational attainment. For each year, we also calculated the proportion of adolescents and young adults with dual diagnosis (incident cases) who were undergoing treatment in the substance use services. We did this in two ways: 1) the proportion of adolescents and young adults with dual diagnosis who were already enrolled in substance use treatment services by the time of their first dual diagnosis contact with the mental health services, and 2) the proportion enrolled in substance use treatment services within a year after their first dual diagnosis contact with the mental health services. Data was analyzed using SAS version 9.4. \u0026nbsp; \u0026nbsp;\u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp; \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eEthical approval\u003c/p\u003e\n\u003cp\u003eThe project was approved by the Regional Data Protection Agency (no. P-2022-616). No further ethical approval is required by Danish Law for studies that are entirely based on register data. \u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eFigure 1a shows that the number of prevalent cases of dual diagnosis among adolescents and young adults has increased remarkably during the study period. The number of prevalent cases increased by 57% between 2010 (1856 cases) and 2020 (2919 cases), followed by a slight declining trend between 2020 and 2022. It also shows that the number of incident cases rose significantly between 2010 and 2017, from 1176 to 1563 new cases. From 2017 and onwards, however, there is a declining trend in the number of new cases, excluding the peak in 2019 which should be interpreted with caution due to data breach in the NPR between 2018 and 2019 [43].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 1b shows that the number of incident and prevalent cases follows a similar trend over the study period when standardized by age and gender, with standardized rates peaking in 2017 followed by a declining trend. Between 2017 and 2022, the prevalence rate declined from 30.4 to 26.2/10,000 and the incidence rate declined from 15.3 to 9.7/10,000 adolescents and young adults in dual diagnosis treatment. \u0026nbsp;\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 1c shows that incident cases are highest among the 22\u0026ndash;25-year-olds and lowest among the 12-17-year-olds during the entire period. However, there is a difference in trends among the three age groups between 2010 and 2017, where we see a distinct increase in incident cases among the 12-17-year-olds compared to the two other age groups. Mean age for incident cases is around 20 during the entire study period.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 1d shows that the number of incident cases is remarkably higher among males than females during the entire period, with the number of new cases ranging between 561 in 2022 as the lowest and 1011 in 2015 as the highest among males, and between 396 in 2018 as the lowest and 577 in 2017 as the highest among females.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 1e shows that schizophrenia, schizotypal, and delusional disorders (F20-29) as well as neurotic, stress-related, and somatoform disorders (F40-49) contribute most to the number of new cases during the entire study period. The number of new cases remains relatively consistent across diagnostic groups, except for neurotic, stress-related, and somatoform disorders (F40-49), where we see a distinct increase in incident cases between 2010 and 2017. However, this increase is followed by a decline between 2017 and 2022. From 2017, there is also a remarkable decline in incident cases of disorders of adult personality and behaviour (F60-69) and schizophrenia, schizotypal and delusional disorders (F20-29).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 1f shows that cannabis related disorders (F12) are by far the most common SUD during the entire study period, as they are present in around 50-60% of new cases of dual diagnosis. It also shows that there is a rise in cocaine related disorders and disorders related to other stimulants (F14-15), from 7% of new cases in 2010 to 13% of new cases in 2022. We also see a slight increase in alcohol related disorders (F10) with a peak in 2022, where they are present in around 24% of new cases of dual diagnosis. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 1g shows that across the period, 12% to 18% of adolescents and young adults were already enrolled in substance use treatment at their first dual diagnosis contact with the mental health services, with an increasing trend towards the end of the study period. Of those who were not already enrolled in substance use treatment, 17% to 24% enrolled in substance use treatment within a year after their first dual diagnosis contact with the mental health services. Here, we see an increasing trend between 2010 and 2020, followed by a steep decline, with the lowest numbers in 2022. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 1h shows that there has been a slight increase in the number of adolescents and young adults above the age of 16 with dual diagnosis who have completed primary school. Nonetheless, despite the increasing trend, the number of adolescents and young adults with dual diagnosis who have not completed primary school remains stable at 7-8% from 2017 and onwards.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eFigure 2a shows a distinct increase in cannabis related disorders (F12) among adolescents and young adults with schizophrenia, schizotypal and delusional disorders (F20-29), while figure 2c shows a slight increase among those with neurotic, stress-related and somatoform disorders (F40-49). On the contrary, figure 2b and 2d show that cannabis related disorders (F12) are declining among adolescents and young adults with mood (affective) disorders (F30-F39), and behavioural and emotional disorders with onset usually occurring in childhood and adolescence (F90-F98). Figure 2b and 2d also show a distinct increase in alcohol related disorders (F10) among adolescents and young adults with mood (affective) disorders (F30-F39), and behavioural and emotional disorders with onset usually occurring in childhood and adolescence (F90-F98). For all four groups of psychiatric diagnoses (figures 2a-d), we see a rise in disorders related to cocaine and other stimulants (F14-15) across the study period. \u0026nbsp; \u0026nbsp;\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eIn this study, we found that while the incidence and prevalence of dual diagnosis among adolescents and young adults increased substantially during the period 2010-2017, there was a declining trend from 2017 to 2022, for incident cases in particular. When distributed by age and gender, the absolute number of new cases remained highest among the oldest age group (22-25 years) and lowest among the youngest age group (12-17 years), and higher among males compared to females during the study period. Further, when incident cases were distributed by the combination of psychiatric diagnoses and SUDs, we found: a distinct increase in cannabis related disorders (F12) among adolescents and young adults with schizophrenia, schizotypal and delusional disorders (F20-29); a distinct increase in alcohol related disorders (F10) among adolescents and young adults with mood (affective) disorders (F30-F39), and behavioural and emotional disorders with onset usually occurring in childhood and adolescence (F90-F98); and a rise in disorders related to cocaine and other stimulants (F14-15) across the four most frequently occurring psychiatric diagnoses. We found an increase in the proportion of adolescents and young adults with dual diagnosis who were enrolled in substance use treatment at the time of their first dual diagnosis contact with the mental health services. Among those who were not yet enrolled in substance use treatment, we found an increase followed by a decrease towards the end of the period in the proportion who enrolled in substance use treatment within a year after their first dual diagnosis contact with the mental health services. Finally, we found a slight increase in the number of adolescents and young adults with dual diagnosis above 16 years of age who had completed primary school (9\u003csup\u003eth\u003c/sup\u003e grade) during the period. However, the proportion who had not completed primary school remained at persistently concerning levels.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eComparison with other studies\u003c/p\u003e\n\u003cp\u003e\u003cem\u003ePotential underdetection of dual diagnosis among adolescents and young adults\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eIn line with global tendencies [1], an increasing number of adolescents and young adults in Denmark are diagnosed with a mental illness and self-report poor mental health [22, 44]. A recent study from Denmark found increasing rates of mental illness among young people from the first lockdown during the Covid-19 pandemic in 2020 to 2022 [45]. Based on this we would have expected to find a substantial increase in the number of new cases of dual diagnosis during the same period in this study, both regarding overall numbers and, in particular, among the youngest age groups (ages 12-17). On the contrary, we found that the number of new cases increased between 2010 and 2017 followed by a declining trend, and that the number of new cases remained lower in the youngest age group compared to the older age groups. The decline after 2019 could be explained by a general decline in substance use among youth in Denmark during the Covid-19 pandemic [46]. Recent national data also shows a decline in young people registered in drug treatment services in Denmark between 2016-2022 [47]. On the contrary, in the same period, there was an increase in the number of young people registered in alcohol treatment services [48].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe declining trend from 2017 and onwards could also be related to the general increase in mental illness among adolescents and young adults in Denmark: with increasing pressure on the mental health system, it is likely that SUDs may not have been systematically screened for, diagnosed and reported, hence contributing to the decline in dual diagnosis occurrence. Persistent underdiagnosis of dual conditions has been reported in other studies [26], which may also help explain the trend across the entire study period: from 1176 new cases in 2010 to 1007 in 2021. Another possible explanation for the declining trend relates to the extensive implementation of new electronic health record- and register systems in the Danish mental health services in 2016-2019, which may have affected reporting practices and hence the diagnostic data [49]. Despite the decreasing trend in the number of new cases, we found a general rise in prevalent cases during the study period, with a far less distinct decrease towards the end of the period compared with incident cases. This indicates a tendency toward longer treatment durations, which may be interpreted in two ways: that mental health services are increasingly more successful in maintaining adolescents and young adults in treatment, or, more concerningly, a tendency of increasing readmissions to mental health services. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eConsistent gender differences in dual diagnosis\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThe international literature points toward a ubiquitous cross-cultural gender gap in youth mental health, with girls and young women having worse average mental health than boys and young men [50, 51]. However, young men are disproportionally affected when it comes to\u0026nbsp;SUDs\u0026nbsp;[2], which may explain our findings of consistent gender differences in dual diagnosis incidence across the entire study period. A recent report on substance use among women across European countries finds that women more often than men are stigmatized on the basis of their substance use, which, in combination with discriminatory and gender-blind services, may deter help-seeking [52]. The same report also highlights that women with dual diagnosis are more likely to be reported as having a SUD subsequent to a primary mental illness, indicating that exclusion from substance use services may occur more frequently among women than men\u0026nbsp;[52]. Another recent study points to similar barriers to accessing care among women with dual diagnosis, and advocates for gender-based perspectives to be applied in studies and treatment of dual diagnosis\u0026nbsp;[53]. While these findings are not specific to adolescents and young adults, they point toward a potential under-detection of dual diagnosis among girls and young women, which may contribute to explain the consistent gender differences in dual diagnosis incidence found in this study. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003ePersistently concerning levels of educational disengagement\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eThe Danish welfare system is characterized by universal and free access to education, including compulsory engagement in primary school up to 9\u003csup\u003eth\u003c/sup\u003e grade. Nevertheless, our findings indicate persistent challenges with educational engagement among adolescents and young adults with dual diagnosis compared with the general population: a recent study with data from 2021 shows that only 0.5% of all young adults in Denmark (ages 18-24) had not completed primary school [54]. Other recent studies with data from Denmark and Norway found an educational gradient in years lived with disability from mental illness and SUDs [55] as well as an association between self-reporting low social status in school and lack of support from teachers or classmates and later depressive symptom trajectories [56]. International evidence confirms and nuances this persistent challenge: a systematic review on school drop-out found a strong association between early life externalizing disorders (including SUDs) and lower educational attainment, while internalizing disorders on the other hand were reported as a consequence of school drop-out [57].\u0026nbsp;Considering this evidence, our findings indicate that not finishing primary school may be an early sign of poor mental health which has not yet been detected. This underscores the need for early interventions to prevent the exacerbation of mental illness symptoms and substance use as well as the socioeconomic consequences associated with not finishing primary school.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cem\u003eThe global drug market and trends in substance use\u0026nbsp;\u003c/em\u003e\u003c/p\u003e\n\u003cp\u003eIn line with global tendencies and recent findings from Denmark showing an increase in use of non-prescribed opioids among adolescents and young adults [58], we would have expected a rise in new cases of dual diagnosis related to opioid use. Instead, we found a stable trend with a continuously low proportion of new cases. While both cannabis related disorders (F12) and schizophrenia, schizotypal, and delusional disorders (F20-29) seem to be declining among adolescents and young adults with dual diagnosis towards the end of the study period, we found a notably distinct rise in cannabis related disorders (F12), when looking only at new cases of dual diagnosis related to schizophrenia, schizotypal and delusional disorders (F20-29). This rise may be interpreted as caused by the increased potency of cannabis, as shown in recent studies from Denmark [59, 60]. Similarly, our findings showing an increase in new cases of dual diagnoses related to cocaine and other stimulants (F14-15) may be explained by the normalization of cocaine use among young people in European countries, which is emphasized in recent literature [61]. \u0026nbsp;\u003c/p\u003e\n\u003cp\u003eStrengths and limitations\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIn our identification of adolescents and young adults with dual diagnosis, we only included those who had a SUD and a mental illness within the same course of treatment in the public mental health services. Furthermore, only three ICD-10 subdivisions of SUDs were deemed relevant in the definition of dual diagnosis: harmful use (F1x.1), dependence syndrome (F1x.2), and withdrawal state (F1x.3). While this narrow definition ensures certain reporting of the development in the number of adolescents and young adults with dual diagnosis who are treated in the mental health services, it comes at the expense of potentially underreporting dual diagnosis occurrence. Only using data from the NPR also contributes to a possible distorted view of dual diagnosis occurrence among adolescents and young adults in the general population, as demonstrated by M\u0026aring;rtensson et al [26], because the NPR only includes data on those who seek help in the mental health services. Relatedly, we linked data based on personal identification numbers, and consequently, adolescents and young adults without such number were not included in the analysis, i.e., undocumented migrants treated in emergency wards. However, the use of data exclusively from the comprehensive NPR is also the main strength of this study, as it covers all mental health services on a national scale. As mentioned, however, the data breach in the NPR between 2018 and 2019 is a limitation to our analysis [49]. Another limitation relates to the calculated decline in incident cases between 2021 and 2022, which may be less distinct than reported here, due to possibly incomplete data from 2022, where treatment courses that were still ongoing at the time of data retrieval were not included in the data set. Finally, it is a limitation to our analysis that we were only able to report on binary (M/F) gender differences, despite recent research shedding light on the disproportionate mental health and substance use problems among transgender, non-binary, and gender diverse youth [62, 63].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eResearch implications and clinical relevance\u003c/p\u003e\n\u003cp\u003eWhile there is strong international evidence showing a ubiquitous decline in mental health including a rise in SUDs among adolescents and young adults, few studies report on the co-occurrence of mental illness and SUDs, not least on the time trends in such co-occurrences. Our study thus significantly contributes to the understanding of recent trends in dual diagnosis occurrence among adolescents and young adults. We find a declining trend in incident cases towards the end of the study period (2017-2022), which, as positive as this trend may seem, could be the result of persistent underdiagnosis. We therefore suggest further research into time trends in dual diagnosis occurrence among adolescents and young adults as well as into the factors that may influence the potential underdetection among this particularly vulnerable group.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eIntegration of substance use services and mental health services is widely recognized as a means to ensure the most efficient treatment of dual diagnosis [64, 65]. In Denmark, integrated treatment is now gradually introduced in the mental health services, and it is expected to be fully implemented on a national scale in 2027 [66]. The success of such large-scale mental health intervention relies on \u0026ndash; among other things \u0026ndash; sufficient and updated knowledge about the people it intends to help. Hence, our findings on recent trends in dual diagnosis among adolescents and young adults provide a timely contribution to further qualify integrated treatment efforts, and tailor treatment to the needs of adolescents and young adults with dual diagnosis. \u0026nbsp;\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eEthical approval\u003c/h2\u003e\n\u003cp\u003eThe project was approved by the Regional Data Protection Agency (no. P-2022-616). No further ethical approval is required by Danish Law for studies that are entirely based on register data.\u003c/p\u003e\n\u003ch2\u003eFunding\u003c/h2\u003e\n\u003cp\u003eThis study was funded by the Novo Nordisk Foundation under grant no. NNF22OC0077756. The Novo Nordisk Foundation is an independent, Danish enterprise foundation that supports scientific, humanitarian and social causes. The Foundation has neither been involved nor consulted in any steps of the research process.\u003c/p\u003e\n\u003ch2\u003eCompeting interests\u003c/h2\u003e\n\u003cp\u003eThe authors have no competing interests to declare that are relevant to the content of this article.\u003c/p\u003e\n\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\n\u003cp\u003eAll authors contributed to the study conception and design. JSJ conducted the data analysis and interpretation. SM revised the data analysis and interpretation. JSJ wrote the main manuscript text, which was reviewed by all authors.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eMcGorry PD, Mei C, Dalal N, Alvarez-Jimenez M, Blakemore S-J, Browne V, et al. The Lancet Psychiatry Commission on youth mental health. The Lancet Psychiatry. 2024;11(9):731-74. https://doi.org/10.1016/S2215-0366(24)00163-9\u003c/li\u003e\n\u003cli\u003eErskine HE, Moffitt TE, Copeland WE, Costello EJ, Ferrari AJ, Patton G, et al. 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Still more people undergoing alcohol treatment 2023 [Available from: https://sundhedsdatastyrelsen.dk/nyheder/2023/stadig-flere-i-behandling-for-alkoholmisbrug.\u003c/li\u003e\n\u003cli\u003eStatistics Denmark. Documentation of statistics for hospitalization 2022. [Available from: https://www.dst.dk/en/Statistik/dokumentation/documentationofstatistics/hospitalization/comparability.\u003c/li\u003e\n\u003cli\u003eCampbell OLK, Bann D, Patalay P. The gender gap in adolescent mental health: A cross-national investigation of 566,829 adolescents across 73 countries. SSM - Population Health. 2021;13:100742. https://doi.org/10.1016/j.ssmph.2021.100742\u003c/li\u003e\n\u003cli\u003eWorld Health Organization, UNICEF. Child and adolescent health in the WHO European Region: adolescent mental health fact sheet. 2024. \u003c/li\u003e\n\u003cli\u003eEuropean Monitoring Centre for Drugs and Drug Addiction. 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Pediatrics. 2021;147(Suppl 2):229-39. https://doi.org/10.1542/peds.2020-023523F\u003c/li\u003e\n\u003cli\u003eMinistry of the Interior and Health of Denmark. Model for et samlet tilbud for mennesker med brug for behandling af b\u0026aring;de en psykisk lidelse og et misbrug (dobbeltdiagnosebehandling) og for oml\u0026aelig;gning af de s\u0026aelig;rlige pladser i psykiatrien. 2021. https://www.ft.dk/samling/20211/almdel/suu/spm/240/svar/1846580/2513707.pdf\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Dual diagnosis, mental illness, substance use disorders, time trends, gender differences, educational attainment","lastPublishedDoi":"10.21203/rs.3.rs-7115166/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7115166/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThis study aims to describe time trends in dual diagnosis occurrence among adolescents and young adults (ages 12\u0026ndash;25) between 2010 and 2022 in Denmark. It also aims to describe how incident cases are distributed by age, gender, diagnoses, educational attainment, and enrollment in substance use treatment services. The study used diagnostic data on every admission to public mental health hospitals in Denmark linked with data from other national social service- and educational registers. Our analyses show a substantial increase in dual diagnosis occurrence in 2010-17, followed by a declining trend. When distributed by gender, age and diagnoses, dual diagnosis was most frequently occurring among males; young adults (ages 22\u0026ndash;25); adolescents and young adults with schizophrenia, schizotypal and delusional disorders; neurotic, stress-related and somatoform disorders; as well as cannabis related disorders across the study period. Our analyses also show an increase in the proportion who were enrolled in substance use treatment at the time of their first dual diagnosis contact with mental health services, but a decreasing trend in the proportion who enrolled within a year after their first dual diagnosis contact. Finally, our analyses show a slight increase in the number of adolescents and young adults with dual diagnosis above 16 years of age who had completed primary school (9th grade) during the study period.\u003c/p\u003e","manuscriptTitle":"Nationwide time trends in co-occurring mental illness and substance use disorders among adolescents and young adults: differences in diagnoses, age, gender, and educational attainment between 2010 and 2022","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-08-27 05:54:55","doi":"10.21203/rs.3.rs-7115166/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"fff13a1f-7233-4402-b319-58971f475f20","owner":[],"postedDate":"August 27th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-09-15T19:23:18+00:00","versionOfRecord":[],"versionCreatedAt":"2025-08-27 05:54:55","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7115166","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7115166","identity":"rs-7115166","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}