Associations between loneliness and outcomes of Common Mental Disorders (CMDs): A systematic review of longitudinal studies

Background Loneliness has been increasingly associated with poor mental health outcomes, yet its prognostic role in people with Common Mental Disorders (CMDs) remains unclear. This systematic review aimed to examine whether loneliness is longitudinally associated with mental health outcomes in individuals with CMDs, regardless of treatment status.

We searched PsycINFO, MEDLINE, Embase and CINAHL from inception to May 2024 for longitudinal studies examining associations between baseline loneliness and CMD outcomes at follow-up. Eligible studies included adults aged 16+ with CMDs. Study quality was assessed using the Newcastle-Ottawa Scale, and certainty of evidence was evaluated using GRADE. Due to heterogeneity in population and outcomes, a narrative synthesis was conducted. The protocol was registered with PROSPERO (registration number: CRD42023410401).

Seventeen studies met the inclusion criteria. Most included studies investigated depression-related outcomes (n=13), with fewer addressing suicidal ideation (n=5), anxiety (n=2), and mixed CMD outcomes (n=1). Several studies contributed data to more than one outcome category. We found that high baseline loneliness was consistently associated with poorer depression-related outcomes at follow-up in people with CMDs. This association was observed across clinical, community, and treatment settings. In contrast, evidence for suicidal ideation and anxiety outcomes was limited and mixed, with inconsistent associations and lower study quality. Only two studies examined treatment outcomes, with mixed findings on whether loneliness influenced intervention response. Overall, the GRADE certainty of evidence was high for depression, low for suicidal ideation, and very low for anxiety and mixed CMD outcomes.

Loneliness appears to be a consistent prognostic factor for poor depression outcomes in people with CMDs, highlighting its potential relevance for assessment and intervention. However, its role in anxiety, suicidality, and treatment response remains unclear. Further high-quality longitudinal research is needed to clarify whether—and under what conditions—loneliness affects broader CMD outcomes and treatment effectiveness. Competing Interest Statement The authors have declared no competing interest. Clinical Protocols https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42023410401 Funding Statement This study did not receive any specific funding. It was conducted as part of the lead author's (TS) PhD, which is funded by the Economic and Social Research Council (ESRC), UK. Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes Data Availability All data produced in the present work are contained in the manuscript and its supplementary information files. https://www.crd.york.ac.uk/prospero/display_record.php?ID=CRD42023410401 List of abbreviations - CMDs - Common mental disorders - NICE - National Institute for Health and Care Excellence - GAD - Generalised anxiety disorder - OCD - Obsessive-compulsive disorder - PTSD - Post-traumatic stress disorder - CORE - Centre for Outcomes and Research and Effectiveness - NOS - Newcastle-Ottawa Scale - RCTs - Randomised Controlled Trials - GRADE - Grading of Recommendations Assessment, Development and Evaluation - PHQ-9 - Patient Health Questionnaire-9 - CES-D-10 - Center for Epidemiologic Studies Depression Scale - CIDI - Composite International Diagnostic Interview - NLP - Natural Language Processing - MDD - Major Depressive Disorder - NESDO - The Netherlands Study of Depression in Older Persons - NESDA - The Netherlands Study of Depression and Anxiety - DBT - Dialectical Behaviour Therapy - ACT - Acceptance and Commitment Therapy - CBT - Cognitive behavioural therapy - BDI-II - Beck Depression Inventory