Association between hair relaxer use and uterine fibroids among women of reproductive age presenting to a national referral hospital in Kenya: a case control study

Association between hair relaxer use and uterine fibroids among women of reproductive age presenting to a national referral hospital in Kenya: a case control study | medRxiv /* */ /* */ <!-- <!-- /*! * yepnope1.5.4 * (c) WTFPL, GPLv2 */ (function(a,b,c){function d(a){return"[object Function]"==o.call(a)}function e(a){return"string"==typeof a}function f(){}function g(a){return!a||"loaded"==a||"complete"==a||"uninitialized"==a}function h(){var a=p.shift();q=1,a?a.t?m(function(){("c"==a.t?B.injectCss:B.injectJs)(a.s,0,a.a,a.x,a.e,1)},0):(a(),h()):q=0}function i(a,c,d,e,f,i,j){function k(b){if(!o&&g(l.readyState)&&(u.r=o=1,!q&&h(),l.onload=l.onreadystatechange=null,b)){"img"!=a&&m(function(){t.removeChild(l)},50);for(var d in y[c])y[c].hasOwnProperty(d)&&y[c][d].onload()}}var j=j||B.errorTimeout,l=b.createElement(a),o=0,r=0,u={t:d,s:c,e:f,a:i,x:j};1===y[c]&&(r=1,y[c]=[]),"object"==a?l.data=c:(l.src=c,l.type=a),l.width=l.height="0",l.onerror=l.onload=l.onreadystatechange=function(){k.call(this,r)},p.splice(e,0,u),"img"!=a&&(r||2===y[c]?(t.insertBefore(l,s?null:n),m(k,j)):y[c].push(l))}function j(a,b,c,d,f){return q=0,b=b||"j",e(a)?i("c"==b?v:u,a,b,this.i++,c,d,f):(p.splice(this.i++,0,a),1==p.length&&h()),this}function k(){var a=B;return a.loader={load:j,i:0},a}var l=b.documentElement,m=a.setTimeout,n=b.getElementsByTagName("script")[0],o={}.toString,p=[],q=0,r="MozAppearance"in l.style,s=r&&!!b.createRange().compareNode,t=s?l:n.parentNode,l=a.opera&&"[object Opera]"==o.call(a.opera),l=!!b.attachEvent&&!l,u=r?"object":l?"script":"img",v=l?"script":u,w=Array.isArray||function(a){return"[object Array]"==o.call(a)},x=[],y={},z={timeout:function(a,b){return b.length&&(a.timeout=b[0]),a}},A,B;B=function(a){function b(a){var a=a.split("!"),b=x.length,c=a.pop(),d=a.length,c={url:c,origUrl:c,prefixes:a},e,f,g;for(f=0;f<d;f++)g=a[f].split("="),(e=z[g.shift()])&&(c=e(c,g));for(f=0;f<b;f++)c=x[f](c);return c}function g(a,e,f,g,h){var i=b(a),j=i.autoCallback;i.url.split(".").pop().split("?").shift(),i.bypass||(e&&(e=d(e)?e:e[a]||e[g]||e[a.split("/").pop().split("?")[0]]),i.instead?i.instead(a,e,f,g,h):(y[i.url]?i.noexec=!0:y[i.url]=1,f.load(i.url,i.forceCSS||!i.forceJS&&"css"==i.url.split(".").pop().split("?").shift()?"c":c,i.noexec,i.attrs,i.timeout),(d(e)||d(j))&&f.load(function(){k(),e&&e(i.origUrl,h,g),j&&j(i.origUrl,h,g),y[i.url]=2})))}function h(a,b){function c(a,c){if(a){if(e(a))c||(j=function(){var a=[].slice.call(arguments);k.apply(this,a),l()}),g(a,j,b,0,h);else if(Object(a)===a)for(n in m=function(){var b=0,c;for(c in a)a.hasOwnProperty(c)&&b++;return b}(),a)a.hasOwnProperty(n)&&(!c&&!--m&&(d(j)?j=function(){var a=[].slice.call(arguments);k.apply(this,a),l()}:j[n]=function(a){return function(){var b=[].slice.call(arguments);a&&a.apply(this,b),l()}}(k[n])),g(a[n],j,b,n,h))}else!c&&l()}var h=!!a.test,i=a.load||a.both,j=a.callback||f,k=j,l=a.complete||f,m,n;c(h?a.yep:a.nope,!!i),i&&c(i)}var i,j,l=this.yepnope.loader;if(e(a))g(a,0,l,0);else if(w(a))for(i=0;i (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];var j=d.createElement(s);var dl=l!='dataLayer'?'&l='+l:'';j.src='//www.googletagmanager.com/gtm.js?id='+i+dl;j.type='text/javascript';j.async=true;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-P4HH5NV'); Skip to main content Home About Submit ALERTS / RSS Search for this keyword Advanced Search Association between hair relaxer use and uterine fibroids among women of reproductive age presenting to a national referral hospital in Kenya: a case control study View ORCID Profile Mary M. Kithikii , View ORCID Profile Marshal M. Mweu , View ORCID Profile Jane W. Muita doi: https://doi.org/10.1101/2025.09.11.25335603 Mary M. Kithikii 1 Department of Public and Global Health, Faculty of Health Sciences, University of Nairobi , Nairobi, Kenya Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Mary M. Kithikii For correspondence: kithikiimary{at}gmail.com Marshal M. Mweu 1 Department of Public and Global Health, Faculty of Health Sciences, University of Nairobi , Nairobi, Kenya Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Marshal M. Mweu Jane W. Muita 1 Department of Public and Global Health, Faculty of Health Sciences, University of Nairobi , Nairobi, Kenya Find this author on Google Scholar Find this author on PubMed Search for this author on this site ORCID record for Jane W. Muita Abstract Full Text Info/History Metrics Data/Code Preview PDF Abstract Uterine fibroids (UFs) precipitate significant morbidity in low and middle income countries, yet evidence on their contextual drivers remains scarce. The objective here was to assess the association between HR use and UFs independent of known risk factors as well as to quantify HR use impact in the population of women of reproductive age. A facility-based case control study was employed to assess the HR-UF association among 264 women presenting to the gynaecologic clinic within Kenyatta National Hospital between 23 rd October and 19 th December 2024. All cases were prospectively recruited while controls were simple randomly sampled from the clinic’s appointment schedule and frequency-matched to cases by day of presentation. A logistic regression model was used to evaluate the HR-UF association, adjusting for the effect of potential confounders. Thereafter, a population attributable fraction (PAF) (along with its confidence interval) was estimated. The odds of UFs among HR users was approximately five times higher than non-users (OR=5.12, 95% CI: 3.00-8.73, P <0.001). This relationship was not confounded by the studied covariates. The PAF estimate for the association was 59.2% (95% CI: 47.2-71.7) – suggesting that about three-fifths of UFs in this population could be prevented if HRs were not used. These findings call for more rigorous enforcement of existing legislations that regulate the supply and use of HRs in the country, with a view to safeguarding consumer health. Introduction Benign gynaecologic conditions (BGC) are leading causes of morbidity globally [ 1 ]. They constitute 5.1% of all years lost to disability; surpassing the cumulative loss due to TB, HIV/AIDS and malaria [ 1 ]. Of all BGCs, uterine fibroids (UFs) remain the most prevalent pelvic tumours that affect women of reproductive age [ 2 ]. UFs, otherwise referred to as leiomyomas/myomas, are defined as benign monoclonal tumours that emerge from the myometrium [ 3 ]. The burden of UFs has seen a 74.4% surge in the last 3o years [ 4 ]. Globally, there was a 60.2% increase in disability-adjusted life years (DALYs) due to UFs [ 5 ]. Black women face the highest burden, with over 80% diagnosed with UFs during their lifetime [ 6 ]. Moreover, recurrence rates post-myomectomy are as high as 59% within a span of five years among women of African descent [ 7 ]. Notable disparities in UF incidence exist in Sub-Saharan Africa (SSA) – Nigeria, Ghana and Kenya reporting incidence rates of 67% [ 8 ], 79% [ 9 ] and 10-20% [ 10 ] respectively. These variations could be attributable to prolonged delays in seeking health care contributing to underreporting [ 11 ]. The use of hair relaxers (HRs) may represent a key UF determinant [ 12 ]. HRs are defined as hair-straightening products designed to break and restructure disulphide bonds found within the cortical layer of the hair shaft allowing it to attain and maintain a straight appearance [ 13 ]. Use of HRs has been linked to an increase in the risk of hormonally-mediated gynaecologic conditions such as uterine, breast and ovarian cancers [ 12 , 14 , 15 ]. This is because HRs serve as primary exposure pathways for endocrine-disrupting chemicals (EDCs) [ 16 , 17 ]. Examples of EDCs found in HRs include formaldehyde (a carcinogen), phthalates, parabens, mercury, lead, triclosan, benzophenone and phenols [ 13 , 18 ]. Racial disparities in exposure to EDCs have been documented [ 19 , 20 ]. Studies report that black women experience higher levels of exposure leading to more adverse health outcomes [ 21 ]. A large prospective cohort study conducted recently in the United States (US) revealed a higher risk of breast cancer in black than white women ascribable to use of chemical hair straighteners [ 15 ]. Studies report that hair products tailored specifically for use by black women contain more EDCs compared to others [ 22 ] and that black women outspend other ethnic groups on hair styling products [ 23 ]. Endocrine disruption and reproductive harm are linked to exposure to more than one EDC [ 18 ]. In SSA, 30-49.2% of black women residing in South Africa [ 24 ] and 59% of Kenyan women use HRs [ 25 ]. Despite their ubiquitous use, to our knowledge, there are no local/regional studies that have sought to evaluate the association between HR use and UFs. Even fewer studies have attempted to describe the frequency and types of HR products used primarily by women in Africa [ 15 ]. Occurrence of UFs is multifactorial in aetiology, with demographic, reproductive, biological and lifestyle factors having been identified [ 26 , 27 ]. Potential lifestyle factors include diet, physical activity, body mass index (BMI), alcohol intake and smoking [ 27 , 28 ]. In particular, alcohol has been shown to exert an oestrogenic effect on the myometrium of the uterus predisposing to UFs [ 29 ]. Reproductive factors include nulliparity, contraceptive use and time since last birth [ 27 ]. Of note, nulliparous women are at a greater risk of UFs compared to parous women – risk reduction being commensurate with the number of children birthed [ 30 , 31 ]. Demographic factors associated with UFs include ethnicity and age [ 27 , 32 ]. The risk of UFs has been shown to increase with age [ 33 ]. Biological risk factors for UFs include family history and age at menarche [ 31 , 34 ]. Early menarche is associated with a heightened risk of UFs [ 31 , 35 , 36 ]. The objective of this study was to quantify the HR-UF association as well as to estimate the impact of HR use in the population of women of reproductive age independent of known risk factors. Materials and methods Study setting and design The study was conducted at Kenyatta National Hospital (KNH) situated in Nairobi County, Kenya. It is a national referral hospital that offers a broad range of medical and surgical services and serves a large catchment population, with approximately 949,000 inpatients and 800,000 outpatients registered annually. This study employed a facility-based case control study design. The rationale for its use was pegged on its suitability for rare outcomes investigation and ease of recruitment of study participants within the facility. This study conformed to the STROBE guidelines for reporting of case-control studies [ 37 ]. Study population The study population comprised primary/referral female patients presenting to the KNH gynaecologic outpatient clinic for care over a three-month study period (23 rd October-19 th December 2024). Outcome definition A case participant was a consenting patient of reproductive age having a confirmed UF diagnosis by ultrasonography. Controls were similarly defined as cases, but without UFs based on ultrasonographic examination. Specifically, they were patients presenting to the same clinic with other gynaecologic conditions not considered related to the primary exposure. As such, patients presenting with adenomyosis, endometriosis, infertility or reproductive cancers (ovarian and uterine) were excluded from the study. Sample size and participant recruitment The requisite sample for the study was determined as described by Kelsey et al . [ 38 ] for case control studies: Where: n 1 = the number of cases; n 2 = the number of controls; p 1 = proportion of cases using hair relaxers; p 2 = proportion of controls using HRs estimated at 59% [ 25 ]. Notably, is the value required for a two-sided 95% confidence interval, Z β = -0.84 is the value of the desired statistical power (80%) and r = ratio of controls to cases set at 1. OR is the odds ratio for the HR-UF association guesstimated at 2. Considering an anticipated non-response rate of 5%, the sample was calculated to 318 women: 159 cases and 159 controls. Given that the gynaecologic clinic registers up to seven cases daily, to meet the required number, all cases presenting on a particular day were prospectively recruited. Controls were simple randomly sampled from the clinic’s patient registry. They were frequency-matched to cases by day of presentation. Study variables HR use – the study’s primary exposure – related to either their direct use or contact (through inhalation or skin) during application. Other factors recorded included the participant’s sociodemographic (age, ethnicity, marital status, level of education, occupation and income level), lifestyle (body mass index (BMI), physical activity, alcohol intake, biomass fuel exposure and co-morbidities such as diabetes and hypertension), reproductive (contraceptive use and parity) and biological (family history and age at menarche) characteristics. These variables were measured using a semi-structured questionnaire coded within the Open Data Kit (ODK) platform ( Table 1 ). Notably, two research assistants were recruited and trained to assist with the data collection exercise. View this table: View inline View popup Table 1. Study variables and their measurements Ethical considerations Approval to conduct the study was granted by the KNH-University of Nairobi Ethics and Research Committee (P487/06/2024) and the National Commission for Science, Technology and Innovation (NACOSTI) (NACOSTI/P/24/41112). Moreover, written informed consent was secured from each individual prior to enrolment in the study. Statistical analysis Initially, the ODK data were exported to R software v4.4.0 for analysis. The R code for these analyses is available as supporting information [ 42 ]. For descriptive statistics, qualitative variables were summarised using frequencies and percentages; whilst quantitative variables with medians and ranges. The crude association between HR use and UF was assessed using a logistic regression model. To evaluate the potential confounding effect of the study covariates on the HR-UF association, all predictors were originally assessed for their unconditional association (amongst HR unexposed women) with UF at a 5% significance level. At this stage, for ease of analysis and owing to a scarcity of observations for some variable categories, recategorisation was effected. Specifically, marital status was regrouped into two categories (single or married), ethnicity was recategorised into five classes (Kikuyu, Luhya, Luo, Kamba and others), level of education was reclassified into three groups (primary, secondary and tertiary levels), income level was dichotomised (low (≤23,670) or high income (>23,670)), alcohol intake converted into a binary variable (no/yes) and BMI was regrouped into four categories (underweight (<18.5 kg/m 2 ), normal weight (18.5 – 24.9 kg/m 2 ), overweight(25.0 - 29.9 kg/m 2 ) or obese (≥ 30.0 kg/m 2 )) [ 43 ]. Significant covariates from this step were then screened for an association (amongst controls) with HR use at P <0.05. Qualifying variables from this latter stage were considered potential confounders of the HR-UF association and, consequently, added to a multivariable logistic model to adjust for their confounding effect. Importantly, only variables that resulted in a >20% change in the coefficient for HR use following a backward stepwise elimination process were retained in the final multivariable model [ 44 ]. The population attributable fraction (PAF) (the proportion of UFs in the population of women of reproductive age that is attributable to HR use), was estimated as specified by Dohoo et al . [ 44 ]: Where: pd is the proportion of cases using HRs and aOR is the adjusted OR for HR use derived from the multivariable model. The estimate’s 95% confidence interval was bootstrapped as described by Ferguson & O’Connell [ 45 ]. Minimisation of errors and biases To minimise selection bias, recruitment of controls was restricted to patients with conditions not thought to be related to HR use. In order to reduce interviewer bias during elicitation of information from respondents, the research assistants were trained on standardised interviewing techniques. Since differential recall of exposures between cases and controls (hence recall bias) was likely, medical records were referenced to verify reproductive and co-morbidity data. To circumvent data entry errors, the ODK platform was pre-specified with valid values. Results Of the sample of 318 individuals, 264 (159 cases and 105 controls) participated in the study. Among the controls, 24 missed their clinic appointments and 27 declined consent. Descriptive statistics for the study variables are presented in Table 2 . In particular, 73.6% ( n =117) of cases compared with 35.2% ( n =37) of controls used HRs. The median age of cases was 40 years (range: 20 – 49 years) while that of controls was 31 years (range: 15 – 49 years). Evidently, 49.7% ( n =79) of cases had a tertiary-level education compared with 59.1% ( n =62) of controls. Pertaining to contraceptive use, 18.2% ( n =29) of cases reported using contraceptives in comparison with 39.1% ( n =41) of controls. The percentage of respondents with a family history of UFs was 25.8% ( n =41) for cases against 7.6% ( n =8) for controls. View this table: View inline View popup Table 2. Descriptive statistics for the predictors of uterine fibroids among women of reproductive age, Kenyatta National Hospital, Kenya ( n = 264) A description of the types of hair relaxers, frequency and duration of their use is shown in Table 3 . Amongst users, the most commonly applied HRs were TCB (58.4%, n =90), Dark & Lovely (35.7%, n =55) and Venus (17.5%, n =27). A majority of the respondents reported using only one HR product (41.3%, n =109). The median duration of HR use among cases was 3 years (Range: 0.5 – 25.0 years), while amongst controls it was 2 years (Range: 0.2 – 20.0 years). View this table: View inline View popup Table 3. Types, frequency and duration of hair relaxer use among women of reproductive age, Kenyatta National Hospital, Kenya ( n = 154) Results of the crude HR-UF association are displayed in Table 4 . The odds of UFs among HR users were approximately five times higher (OR = 5.12; 95% CI: [3.00; 8.73]) than non-users. View this table: View inline View popup Download powerpoint Table 4. Univariable analysis of the hair relaxer-uterine fibroid association among women of reproductive age, Kenyatta National Hospital, Kenya Table 5 shows the results of the association between the study variables (amongst HR exposure-negative women) and UF. Notably, age, level of education and contraceptive use were significantly associated with UF and thus qualified for screening with HR use. View this table: View inline View popup Table 5. Association between study covariates and uterine fibroids among women of reproductive age, Kenyatta National Hospital, Kenya. View this table: View inline View popup Download powerpoint Table 6. Association between the eligible covariates and hair relaxer use among women of reproductive age, Kenyatta National Hospital, Kenya. Results of the association between qualifying covariates and HR use are presented in Table 6 . Only level of education was associated with HR use at the 5% significance level and was thus eligible for assessment of its potential confounding effect on the HR-UF association in the multivariable analysis. View this table: View inline View popup Download powerpoint Table 6: Multivariable analysis for the association between hair relaxer use and uterine fibroids among women of reproductive age, Kenyatta National Hospital, Kenya. Table 7 presents the results of the multivariable analysis. Since education level did not confound the HR-UF association (i.e. it did not result in a >20% change in the regression coefficient for HR use) the odds ratio for the unadjusted association was retained and subsequently employed in the estimation of PAF. The PAF estimate for this association was 59.2% (95% CI: 47.2 – 71.7). Discussion In this study setting, women predominantly utilised TCB, Dark & Lovely and Venus HR products. Similarly, in Nakuru, Kenya, Dark & Lovely and TCB were the most desirable brands amongst HR users [ 2 ]. These findings underscore the popularity of these products among Kenyan women. The products are available on the market as either lye or no-lye products [ 46 – 48 ]. Lye HRs contain sodium or potassium hydroxide and are preferred over no-lye HRs due to their greater effectiveness in breaking the disulphide bonds in afro hair shafts resulting in straighter hair [ 49 , 50 ]. Nonetheless, Lye HRs are highly alkaline and have been linked to higher scalp irritations, lesions and burns [ 50 ]. No-lye HRs contain calcium hydroxide and guanidine carbonate and are associated with lower irritations or burns [ 50 ]. Notwithstanding, a separate study has demonstrated that both lye and no-lye HRs exhibit similar pH and are equally corrosive to the skin [ 51 ]. The scalp burns, lesions and irritations caused by these HRs may compromise the skin barrier, potentially increasing dermal absorption of chemicals of concern present in HRs and thereby predisposing to UFs [ 52 , 53 ]. This study demonstrated that a number of women were applying multiple products. This integrated use may synergise their effect, further amplifying the risk of UFs. Frequent use of hair-straightening products has been associated with a two-times higher risk of reproductive cancer [ 54 ]. The median duration of HR use for this study population was three years (Range: 0.2, 25.0). Prolonged use of these products could increase exposure to EDCs which could heighten a woman’s risk of UFs. In Ghana, an increasing trend in the long-term usage of no-lye HRs has been observed, with longer duration associated with a higher risk of breast cancer [ 55 ]. Comparably, a positive relationship between duration of HR use and incidence of UFs has been noted elsewhere [ 33 ]. This study registered a strong association between HR use and UFs even after adjustment for potential confounders – HR users having about five times higher odds of UF than non-users. This reveals that HR use is a strong driver for UFs in this catchment population. This finding is corroborated by results from large prospective studies [ 12 , 15 , 56 ]. As a case in point, Chang et al . [ 56 ] demonstrated that women using hair-straightening products had about twice the rate of uterine cancer as non-users. Of note, the HR-UF association in our study yielded a high PAF estimate (59.2%) – signifying that around three-fifths of the UFs in the population could be averted if HRs were not used. This finding calls for dedicated community advocacy campaigns with a view to raising awareness about the potential risks associated with hair-straightening products. A potential mechanism explaining this association is exposure to EDCs commonly found in HRs such as parabens, phthalates, phenols, triclosan, benzophenone and formaldehyde, which are known to influence disease risk [ 13 , 18 , 22 , 57 ]. These hormonally-active chemicals perturb endocrine function, potentially contributing to adverse reproductive health outcomes, including UFs [ 12 , 22 , 58 ]. Indeed, high concentrations of phthalates and paraben metabolites have been recovered from urine samples of fibroid cases [ 59 – 62 ]; connoting a potential causal role [ 63 – 66 ]. Regrettably, consumers may not rely on HR product labels to minimize exposure to EDCs as studies report inconsistencies between listed ingredients and actual measured contents [ 22 , 67 ]. This study is not without its limitations. Despite deliberate efforts to minimise recall bias in the design stage, recall of some exposures was still likely to have been more complete for cases than controls, and this would bias effect estimates away from unity. Population-based (as opposed to hospital-based) controls would have been ideal for this study since they provide a more realistic estimate of the frequency of the exposure (HR use) in the catchment population. However, identifying suitable controls in the population comparable to the hospital cases would have been daunting. Consequently, the reliance on hospital-based controls is likely to have overestimated the frequency of HR use in the population, thus biasing the estimated HR-UF association towards unity. Conclusions A strong association between HR use and UFs was uncovered; HR users having about five times higher odds of UF than non-users. This relationship was not confounded by the studied factors. Accordingly, this association yielded a high PAF – implying that about three-fifths of UFs in the population could be averted devoid of HRs. These findings call for stricter enforcement of existing regulatory frameworks for cosmetic products in the country in order to safeguard consumer health. Data Availability Data are available at Harvard Data verse [Kithikii MM. Replication Data for: Association between hair relaxer use and uterine fibroids among women of reproductive age presenting to a national referral hospital in Kenya: A case control study. Harvard Dataverse 2025. doi:10.7910/DVN/N6YJNG] https://doi.org/10.7910/DVN/N6YJNG Acknowledgments We are indebted to the hospital’s administration for graciously permitting the conduct of this study. 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Environmental Health Perspectives . 2012 ; 120 : 935 – 943 . doi: 10.1289/ehp.1104052 OpenUrl CrossRef PubMed Web of Science View the discussion thread. Back to top Previous Next Posted September 15, 2025. Download PDF Data/Code Email Thank you for your interest in spreading the word about medRxiv. NOTE: Your email address is requested solely to identify you as the sender of this article. Your Email * Your Name * Send To * Enter multiple addresses on separate lines or separate them with commas. You are going to email the following Association between hair relaxer use and uterine fibroids among women of reproductive age presenting to a national referral hospital in Kenya: a case control study Message Subject (Your Name) has forwarded a page to you from medRxiv Message Body (Your Name) thought you would like to see this page from the medRxiv website. 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