Use of Chemical Hair Straighteners and Menstrual Disturbances

Abstract

Background:

Use of chemical hair straighteners (“relaxers”) is associated with higher risks of hormonally mediated conditions. We hypothesized users of relaxers would have a higher prevalence of abnormal uterine bleeding (AUB) and dysmenorrhea.

Materials and Methods:

We analyzed baseline data from Pregnancy Study Online, an internet-based preconception cohort study of North American pregnancy planners. We included 14,366 participants aged 21–39 years who enrolled during 2014–2024 and reported on their typical menstrual cycle characteristics when not using hormones. We collected data on history of use, age at first use, frequency per year, duration of use, and number of burns. We defined AUB as cycle length <24 or >38 days, flow ≥ 7 days, irregular cycles, and/or heavy flow (>30 pads/tampons per menses). We defined dysmenorrhea as severe cramps requiring medication and bed rest. We used modified Poisson regression models to estimate prevalence ratios (PRs) and 95% confidence intervals (CIs) for the associations of relaxer use with AUB and dysmenorrhea, adjusted for potential confounders.

Results:

Overall, 2% of participants were current users and 10% were former users of relaxers. The frequencies of AUB and dysmenorrhea were 31% and 8%, respectively. PRs for current (versus never) relaxer use were 1.16 (95% CI: 1.00–1.33) for AUB and 1.30 (95% CI: 0.94–1.80) for dysmenorrhea. Participants who reported ever experiencing burns (versus never use) had a higher prevalence of dysmenorrhea (1–4 burns: PR = 1.42, 95% CI: 1.04–1.93; ≥5 burns: PR = 1.46, 95% CI: 1.00–2.13).

Conclusion:

Relaxer use was associated with a higher prevalence of menstrual disturbances.

Keywords

menstruation disturbances dysmenorrhea personal care products hair products chemical straighteners epidemiology

Introduction

Chemical hair straighteners (“relaxers”) are used widely, particularly by Black women. In the United States, the lifetime prevalence of relaxer use ranges from 52% to 94% among Black women,^1–7 15–45% among Hispanic women and those of other minoritized racial and ethnic groups,^2,5,6 and ∼5% among non-Hispanic White women.^3,6 Racial and ethnic patterns of relaxer use occur in the context of Eurocentric beauty norms and race-based hair discrimination, which create social and economic pressures to straighten hair.^2,8–12

Types of relaxers include lye-based, non-lye (active ingredients: lithium or potassium hydroxide, guanidine carbonate, thioglycolate, and others), and keratin.^13,14 Relaxers can contain harmful constituents, including endocrine-disrupting chemicals (EDCs; e.g., phthalates, parabens, bisphenols),¹⁵ formaldehyde and chemicals that release formaldehyde when heated during application,^14,16 and toxic metals.¹⁷ Chemicals from the application of relaxers may enter the body through dermal absorption and inhalation.¹⁸ Because scalp skin is particularly permeable to chemicals,¹⁹ use of relaxers may lead to higher internal exposure relative to many other types of personal care products. Furthermore, burns, lesions, and inflammation of the scalp that can occur during application may facilitate greater internal exposure to hair product chemicals.^20,21

Epidemiological studies have identified positive associations of relaxer use with hormone-responsive health outcomes, including early menarche,^22,23 subfertility,⁶ uterine leiomyomata (UL),^24,25 breast cancer,^3,26–29 and gynecological cancers.^30–32 However, no epidemiological study has investigated the association between personal use of relaxers and menstrual cycle characteristics during adulthood. Menstruation is a sensitive marker of reproductive function with important effects on fertility, overall health, and quality of life.^33–36 Abnormal uterine bleeding (AUB), defined as bleeding that is abnormal in terms of volume, duration, frequency, and/or regularity, has a lifetime prevalence of 10–30% among menstruating individuals.^35,37 Approximately 16–29% of menstruating individuals experience dysmenorrhea (menstrual pain and cramping) that interferes with daily activities, with resultant impacts on quality of life, academic performance, and work productivity.^38,39

We estimated the associations between relaxer use and the prevalence of AUB and dysmenorrhea in a cohort of individuals who were trying to conceive. We hypothesized that current and former use of relaxers would be associated with higher prevalence of menstrual disturbances, and that we would observe stronger associations for users with higher lifetime relaxer use.

Materials and Methods

Study design

We analyzed data from Pregnancy Study Online (PRESTO), an internet-based preconception, prospective cohort study (June 2013–ongoing).⁴⁰ Eligible participants are aged 21–45 years, residing in the United States or Canada, trying to conceive with a male partner, and not using contraceptives or fertility treatment. Participants provided detailed information through a structured questionnaire at baseline. PRESTO is approved by the Boston University Medical Campus Institutional Review Board. All participants provided informed consent.

Exposure assessment

Assessment of relaxer use in PRESTO has been described previously.⁶ Starting in June 2014, on the baseline questionnaire, we included questions adapted from the Black Women’s Health Study^24,41 about history of using a “chemical hair straightener (e.g., relaxer)” (never, current, former, do not know). Among current and former users, we queried age at first use, frequency of use per year, total number of years of use, the number of applications that included any burns, and the brand of chemical hair straightener used the longest (Supplementary Appendix SA1). Starting in August 2018, we added “chemical perms” to the questionnaire items to capture all straightening procedures, because “perm” is sometimes used colloquially to refer to straightening procedures,⁴² albeit the chemicals in perms to curl hair differ from those found in chemical straighteners.⁴³ (The most common active ingredient in chemical curling procedures is ammonium thioglycolate.⁴³) We asked participants to specify whether they had ever used the following product types: “chemical hair straightener (e.g., relaxer),” “chemical perm,” “both chemical hair straighteners (e.g., relaxer) and chemical perms,” or “not sure.”

Outcome assessment

On the baseline questionnaire, participants reported on their typical menstrual cycle characteristics during the past couple of years when not using hormones (Supplementary Appendix SA2). We defined AUB by aligning PRESTO items with the International Federation of Gynecology and Obstetrics (FIGO) definition as self-report of irregular cycles, short or long cycles (<24 or >38 days, respectively), long duration of flow (≥7 days, versus FIGO’s ≥8 days), or heavy flow (>30 pads/tampons per period).³⁷ We defined dysmenorrhea as self-reported severe cramps requiring medications and bed rest (Supplementary Appendix SA3).

Covariate assessment

On the baseline questionnaire, participants reported sociodemographic characteristics (age, race and ethnicity, educational attainment of the participant and their parents/caregiver, household income), reproductive history (parity, history of infertility [ever trying for ≥12 months to become pregnant without success], ever diagnosed with polycystic ovary syndrome [PCOS], endometriosis, or UL, pregnancy attempt time at study entry), anthropometric factors (weight and height), behaviors (physical activity, smoking, alcohol and caffeine intakes, multivitamin use), and mental health measures (diagnoses of anxiety or depression, perceived stress,⁴⁴ depressive symptoms⁴⁵). We calculated body mass index (BMI) as weight (kilograms) divided by height (meters) squared.

Participants

We included PRESTO participants aged 21–39 years, who enrolled between June 2014 and November 2024 (n = 20,078; Supplementary Fig. S1). We excluded participants aged 40–45 years (n = 513) to reduce variability in menstrual cycle characteristics due to perimenopause. We excluded participants whose last menstrual period date at baseline was >1 year prior at study entry (n = 449) or implausible (i.e., after the date of questionnaire completion; n = 13), those with >60 days elapsed between screening and baseline (n = 49), and those who could not characterize cycle regularity during the past couple of years because of hormone use (n = 4,676) or did not respond to this item (n = 12). The analytic cohort included 14,366 participants.

Statistical analysis

We characterized exposure to relaxers according to history of use (current, former, never) and measures of lifetime use (age at first use, number of times burned, duration of use, frequency of use, and cross-classified duration and frequency of use). We did not analyze the longest-used brand because it was missing for 76% of ever users. Among participants who reported ever using “relaxers or chemical perms,” 31% used relaxers only, 57% used perms only, 9% used both, and 3% were not sure. We categorized participants who reported use of relaxers only or both relaxers and perms as exposed and categorized participants who reported use of perms only or were not sure as never-users of relaxers.⁶

We assessed participant characteristics by history of relaxer use, overall and stratified by race and ethnicity (Black, non-Black minoritized racial and ethnic groups, non-Hispanic White). We considered race and ethnicity as sociopolitical constructs reflecting exposure to racism.^46,47 Black participants included those who were non-Hispanic Black, Hispanic Black, or multiracial Black. Non-Black participants of minoritized racial and ethnic groups included those who identified as Alaska Native, American Indian, Asian, Hispanic (ethnicity), Indigenous, multiracial, Native Hawaiian, Pacific Islander, or another race and did not identify as Black.

We used modified Poisson regression models to estimate prevalence ratios (PR) and 95% confidence intervals (CI) for the associations of relaxer use with AUB and dysmenorrhea, with never users as the referent group for all comparisons. We chose modified Poisson regression models as an alternative to log-binomial regression models to avoid convergence problems that can occur with log-binomial models. We assessed measures of lifetime relaxer use among current and former users combined and in separate analyses of current and former users. We identified potential confounders using a directed acyclic graph (Supplementary Fig. S2). We constructed two sets of multivariable models. The minimally adjusted model controlled for age (21–24, 25–29, 30–34, 35–39 years) as a potential confounder and enrollment year (2014–2015, 2016–2020, 2021–2024) as a precision variable. The fully adjusted model controlled for age, enrollment year, race and ethnicity, parent’s education (highest completed by mother, father, or other primary caregiver: ≤ high school, some college, ≥ college), history of anxiety or depression diagnosis (yes, no), participant’s education (≤ high school, some college, college, graduate school), and current smoking (yes, no).

We estimated PRs for the associations of relaxer use with separate AUB symptoms: irregular cycles, duration of flow ≥7 days, and heavy menstrual bleeding. We did not assess the associations of relaxer use with short or long cycles due to the small numbers of exposed cases.

In sensitivity analyses, we excluded participants who started using relaxers within ≤5 years to further ensure that relaxer use preceded the outcome measures. We analyzed the use of relaxers or perms as a combined exposure to accurately classify exposure status for participants who interpreted “perms” to mean chemical straighteners. To reduce the potential for selection bias, we excluded participants with ≥6 cycles of pregnancy attempt time or a history of infertility before enrollment, subgroups in whom participation in PRESTO may be influenced by subfertility. Separately, we excluded participants with ≥12 cycles of pregnancy attempt time at enrollment because these participants may be more likely to change their relaxer use in response to concerns about subfertility. An epidemiological study of the association between relaxer use and uterine cancer (published on October 17, 2022) and subsequent related lawsuits received extensive media coverage.^31,48–50 Therefore, to reduce the potential for selection bias (i.e., if concern about the health effects of relaxer use was a stronger determinant of PRESTO participation after the publication), we conducted a sensitivity analysis excluding participants who enrolled after October 17, 2022. Finally, to assess the influence of gynecological conditions on our results, we excluded participants with a history of diagnosed PCOS, endometriosis, or UL.

We stratified by race and ethnicity to account for differences in relaxer use. We stratified by age (21–29, 30–39 years), hypothesizing that older participants would be more susceptible to the harmful effects of toxicants due to ovarian aging. We stratified by BMI (<30, ≥30 kg/m²), using BMI as a proxy for adiposity, because some EDCs found in relaxers (e.g., galaxolide, linolool) are lipophilic,^15,51,52 and the sequestering of lipophilic chemicals in adipose tissue may result in lower bioavailability of the chemicals among individuals with greater adiposity.

Missing data

We used fully conditional specification methods to multiply impute missing data for relaxer use, covariates, and menstrual cycle characteristics. Missingness ranged from 0% (e.g., age) to 4% (parent’s education). Across racial and ethnic groups, missingness was 2% to 3% for history of relaxer use and <1% for menstrual cycle characteristics. We imputed 20 datasets and combined estimates and standard errors across datasets using Rubin’s rule.

Results

Participant characteristics

Among 14,366 participants, 2% were current users and 10% were former users of relaxers (Table 1). Mean age was 30 years, and 69% had a college or graduate degree. Current users of relaxers had lower average educational attainment and household income, higher average BMI, and a higher prevalence of current smoking, compared with former and never users. Current users of relaxers also had a higher prevalence of history of infertility, a longer pregnancy attempt time at enrollment, higher perceived stress, and more depressive symptoms. Black participants had the highest prevalence of current (18%) and former (56%) use of relaxers (Supplementary Table S1).

Table 1.

Participant Characteristics According to History of Relaxer Use^a Among Participants in Pregnancy Study Online, 2014–2024 (n = 14,366)

Characteristic	Overall	Current	Former	Never
Number	14,366	330	1,485	12,551
Age at baseline (years), mean	30.4	30.7	31.1	30.3
Age at baseline, %
21–24	8	9	6	8
25–29	34	32	30	35
30–34	41	35	42	41
35–39	17	24	22	16
Geographic region of birth, %
Northeastern U.S.	19	16	20	19
Southern U.S.	21	29	27	20
Midwestern U.S.	23	17	21	24
Western U.S.	15	11	11	16
Canada	14	7	7	14
Other countries	8	20	14	7
Geographic region of current residence, %
Northeastern U.S.	18	19	19	18
Southern U.S.	27	40	37	25
Midwestern U.S.	22	17	21	23
Western U.S.	17	13	15	18
Canada	15	11	9	16
Race and ethnicity, %
Black^b	7	53	38	2
Non-Black participants of minoritized racial and ethnic groups^c	15	17	18	15
Non-Hispanic White	78	30	44	83
Parent’s education, %
≤High school	18	31	22	17
Some college	26	27	29	26
College or graduate school	55	42	49	57
Education, %
≤High school	8	22	10	7
Some college	23	34	28	22
College	32	23	28	33
Graduate school	37	21	34	38
Household income (U.S. dollars/year), %
<50,000	22	47	29	21
50,000–99,999	33	27	32	33
100,000–149,000	24	12	19	25
≥150,000	21	13	19	22
Employed, %	84	79	83	84
Private health insurance, %	78	61	76	79
Body mass index (kg/m²), mean	28.7	30.9	30.1	28.4
Body mass index (kg/m²), %
<18.5	2	3	1	2
18.5–24.9	39	29	33	41
25–29	24	20	23	24
≥30	35	48	42	34
Current smoker, %	11	26	14	10
Parous, %	36	39	36	36
History of infertility, %	22	42	29	20
Last method of contraception, %
Oral contraceptives	23	24	24	23
Other hormonal methods	5	12	6	5
Barrier methods	43	29	38	44
Natural methods	29	35	32	29
Cycles of attempt time at study entry, %
0–2	49	28	43	51
3–5	22	25	20	22
6–11	15	18	16	15
≥12	13	28	20	12
Pregnancy during the past 4 years, %	43	34	39	43
Diagnoses of medical conditions, %
Anxiety	28	18	27	29
Depression	26	20	26	27
PCOS	12	17	16	12
Endometriosis	3	3	4	3
UL	3	4	5	3
Perceived Stress Scale-10 item score, mean^d	16.8	18.4	17.3	16.7
Major Depression Inventory score, mean^e	12.3	15.9	13.6	12.0
Caffeine intake, mg/day, mean	128.7	106.9	113.6	131.3
Alcohol intake, drinks/week, mean	2.7	2.8	2.8	2.7
Physical activity, total Metabolic Equivalent of Task hours/week, mean	34.2	33.4	33.1	34.3
Ever use of perms^a,f, %	19	24	23	19
Occupational exposure to hair product chemicals, %	0.3	1	0.7	0.2
Abnormal uterine bleeding, %	31	45	34	30
Dysmenorrhea, %	8	14	11	8

Data are from the first multiply imputed dataset. All characteristics except age are age-standardized to the age distribution of the cohort.

Perm users who also used relaxers are categorized as users of relaxers (current or former, as applicable). Perm users who did not use relaxers are categorized as nonusers of relaxers. Data about use of perms were collected starting in Aug. 2018.

Includes Non-Hispanic Black, Hispanic Black, and multiracial race Black participants.

Includes Hispanic participants of any race, and Alaska Native, American Indian, Asian, Indigenous, multiracial, Native Hawaiian, and Pacific Islander non-Black participants.

Past-4-week lookback period; range 0–40 (higher indicates greater perceived stress).

Past-2-week lookback period; range 0–50 (higher indicates greater depressive symptoms).

Among participants who enrolled after addition of the perm item to the questionnaire.

The frequencies of AUB and dysmenorrhea were 31% and 8%, respectively. AUB and dysmenorrhea were most prevalent among Black participants, followed by non-Black participants of minoritized racial and ethnic groups (Table 2). The most prevalent AUB symptom was irregular cycles.

Table 2.

Prevalence of Menstrual Cycle Characteristics in Pregnancy Study Online, 2014–2024, Overall and Stratified by Race and Ethnicity

		Race and ethnicity
	Overall	Black	Non-Black minoritized racial and ethnic groups	Non-Hispanic white
Number	14,366	1,000	2,149	11,217
Column percentage
AUB	31	42	33	29
Irregular cycles	24	32	26	22
Duration of flow ≥7 days	8	14	8	7
Heavy bleeding (>30 pads/tampons per menses)	4	7	4	4
Short cycles (<24 days)^a	2	4	3	2
Long cycles (>38 days)^a	0.7	1	1	1
Dysmenorrhea	8	14	9	8

Among participants who reported regular cycles.

Associations of relaxer use with AUB

Current (versus never) relaxer use was associated with a higher AUB prevalence (fully adjusted PR = 1.16, 95% CI: 1.00, 1.33; Table 3). Some measures of lifetime relaxer use were weakly, positively associated with AUB prevalence. These associations were driven by current use (Supplementary Table S2). Measures of lifetime relaxer use were not meaningfully associated with AUB prevalence among former users.

Table 3.

Relaxer Use and Prevalence of Abnormal Uterine Bleeding and Dysmenorrhea in Pregnancy Study Online, 2014–2024

	Outcome: abnormal uterine bleeding				Outcome: dysmenorrhea
	N	Cases	Minimally adjustedPR(95% CI)^a	Fully adjustedPR(95% CI)^b	N	Cases	Minimally adjustedPR(95% CI)^a	Fully adjustedPR(95% CI)^b
Never use of relaxers^c	12,551	3,773	1.00 (ref.)	1.00 (ref.)	12,551	1,002	1.00 (ref.)	1.00 (ref.)
Former use	1,485	488	1.10 (1.02, 1.19)	0.96 (0.88, 1.05)	1,485	154	1.31 (1.11, 1.54)	1.06 (0.88, 1.27)
Current use	330	152	1.49 (1.32, 1.70)	1.16 (1.00, 1.33)	330	46	1.79 (1.34, 2.38)	1.30 (0.94, 1.80)
Age at first use, years
<10	310	112	1.20 (1.03, 1.39)	0.89 (0.75, 1.06)^d	310	45	1.89 (1.43, 2.48)	1.36 (0.97, 1.90)^d
10–14	593	218	1.21 (1.08, 1.35)	0.96 (0.85, 1.10)^d	593	74	1.56 (1.24, 1.95)	1.22 (0.93, 1.59)^d
15–19	461	177	1.24 (1.10, 1.40)	1.09 (0.96, 1.24)^d	461	42	1.08 (0.79, 1.46)	0.94 (0.68, 1.28)^d
≥20	451	133	1.03 (0.88, 1.19)	0.98 (0.85, 1.14)^d	451	39	1.16 (0.83, 1.61)	1.10 (0.79, 1.52)^d
No. of times burned during application
0	1,122	347	1.05 (0.95, 1.15)	0.97 (0.88, 1.06)	1,122	93	1.07 (0.87, 1.31)	0.97 (0.78, 1.20)
1–4	451	191	1.38 (1.23, 1.55)	1.06 (0.92, 1.21)	451	71	1.92 (1.52, 2.43)	1.42 (1.04, 1.93)
≥5	242	102	1.36 (1.16, 1.58)	1.05 (0.87, 1.26)	242	36	1.94 (1.42, 2.64)	1.46 (1.00, 2.13)
Duration of use, years
<1	566	182	1.06 (0.94, 1.20)	1.01 (0.89, 1.14)	566	49	1.09 (0.83, 1.44)	1.01 (0.77, 1.33)
1–4	492	138	0.94 (0.81, 1.09)	0.86 (0.74, 0.99)	492	48	1.25 (0.95, 1.66)	1.08 (0.81, 1.44)
5–9	255	106	1.37 (1.19, 1.59)	1.12 (0.95, 1.32)	255	33	1.65 (1.19, 2.28)	1.20 (0.83, 1.74)
≥10	502	214	1.42 (1.27, 1.58)	1.11 (0.96, 1.28)	502	70	1.76 (1.39, 2.23)	1.24 (0.90, 1.69)
Frequency of use, per year
1	898	284	1.07 (0.97, 1.19)	0.98 (0.88, 1.08)	898	78	1.15 (0.92, 1.44)	1.02 (0.81, 1.27)
2	335	117	1.14 (0.97, 1.33)	0.98 (0.83, 1.15)	335	40	1.48 (1.09, 2.01)	1.19 (0.86, 1.64)
3–4	308	116	1.26 (1.09, 1.46)	0.99 (0.83, 1.17)	308	38	1.54 (1.13, 2.11)	1.11 (0.77, 1.61)
≥5	274	123	1.42 (1.24, 1.63)	1.11 (0.94, 1.30)	274	44	1.89 (1.41, 2.53)	1.37 (0.97, 1.93)
Duration (years) × frequency (per year)
<1 year × 1 per year	519	165	1.05 (0.92, 1.20)	1.00 (0.88, 1.13)	519	44	1.10 (0.82, 1.47)	1.02 (0.76, 1.36)
1–4 years × 1 per year	272	71	0.91 (0.74, 1.12)	0.83 (0.68, 1.02)	272	23	1.16 (0.78, 1.73)	1.02 (0.69, 1.52)
≥5 years × 1 per year	107	48	1.56 (1.26, 1.94)	1.25 (1.00, 1.54)	107	11	1.37 (0.77, 2.45)	1.01 (0.56, 1.82)
<5 years × ≥2 per year	267	84	1.01 (0.84, 1.22)	0.93 (0.77, 1.12)	267	30	1.30 (0.90, 1.87)	1.12 (0.77, 1.62)
≥5 years × ≥2 per year	650	272	1.38 (1.25, 1.52)	1.08 (0.95, 1.24)	650	92	1.78 (1.44, 2.18)	1.27 (0.94, 1.71)

Adjusted for age in years (21–24, 25–29, 30–34, 35–39) and year of enrollment (2014–2015, 2016–2020, 2021–2024).

Adjusted for age, year of enrollment, race and ethnicity (Black, non-Black minoritized racial and ethnic groups, non-Hispanic White), parent’s educational attainment (≤ high school, some college, ≥college), history of anxiety or depression diagnosis (yes, no), participant’s educational attainment (≤high school, some college, college, graduate school), and current smoking (yes, no).

Referent group for all comparisons.

Adjusted for age, year of enrollment, race and ethnicity (Black, Non-Black participants of minoritized racial and ethnic groups, Non-Hispanic White), and parent’s educational attainment (≤high school, some college, ≥college).

CI, confidence interval; PR, prevalence ratio.

When analyzing AUB symptoms separately, current relaxer use was associated with a higher prevalence of irregular cycles (Supplementary Table S3). Current use was weakly, positively associated with flow ≥7 days (Supplementary Table S3), but not with heavy menstrual bleeding (not shown). Some measures of lifetime relaxer use were positively associated with irregular cycles and flow ≥7 days.

The association of current (versus never) relaxer use with AUB prevalence was consistent across sensitivity analyses (Supplementary Table S4). Former (versus never) relaxer use was not meaningfully associated with AUB prevalence in sensitivity analyses.

Current relaxer use was positively associated with AUB among non-Black participants of minoritized racial and ethnic groups and non-Hispanic White participants, but not among Black participants, in fully adjusted models (Table 4). Several measures of lifetime relaxer use were positively associated with AUB among non-Black participants of minoritized racial and ethnic groups. Among non-Hispanic White participants, age at first use (15–19 years) and frequency of use (≥5 times per year) were positively associated with AUB. Measures of lifetime relaxer use were not meaningfully associated with AUB among Black participants. Current relaxer use and some measures of lifetime use were positively associated with AUB prevalence among participants aged 21–29 and those with BMI < 30 kg/m² (Supplementary Tables S5 and S6).

Table 4.

Relaxer Use and Prevalence of Abnormal Uterine Bleeding in Pregnancy Study Online, 2014–2024, Stratified by Race and Ethnicity

	Black			Non-Black minoritized racial and ethnic groups			Non-Hispanic white
	N	Cases	PR (95% CI)^a	N	Cases	PR (95% CI)^a	N	Cases	PR (95% CI)^a
Never use of relaxers^b	262	116	1.00 (ref.)	1,820	608	1.00 (ref.)	10,469	3,049	1.00 (ref.)
Former use	560	217	0.90 (0.76, 1.08)	277	81	0.93 (0.77, 1.14)	648	190	0.97 (0.86, 1.10)
Current use	178	88	1.00 (0.81, 1.24)	52	24	1.33 (0.96, 1.84)	100	40	1.21 (0.93, 1.56)
Age at first use, years
<10	242	87	0.81 (0.65, 1.00)^c	22	12	1.60 (1.09, 2.35)^c	46	13	0.96 (0.57, 1.64)^c
10–14	340	143	0.93 (0.77, 1.13)^c	74	30	1.17 (0.86, 1.58)^c	179	45	0.82 (0.63, 1.05)^c
15–19	115	56	1.03 (0.82, 1.30)^c	106	31	0.87 (0.64, 1.19)^c	240	90	1.20 (1.02, 1.42)^c
≥20	41	19	1.00 (0.68, 1.48)^c	127	32	0.82 (0.60, 1.13)^c	283	82	1.05 (0.87, 1.26)^c
No. of times burned during application
0	197	78	0.90 (0.72, 1.12)	272	76	0.91 (0.74, 1.12)	653	193	1.00 (0.88, 1.12)
1–4	322	135	0.93 (0.77, 1.13)	48	26	1.39 (1.03, 1.86)	81	30	1.05 (0.78, 1.42)
≥5	219	92	0.96 (0.77, 1.18)	9	3	0.99 (0.34, 2.88)	14	7	1.25 (0.56, 2.79)
Duration of use, years
<1	52	24	0.99 (0.71, 1.37)	127	38	0.96 (0.73, 1.28)	387	120	1.02 (0.88, 1.18)
1–4	108	38	0.82 (0.62, 1.09)	132	34	0.73 (0.53, 1.01)	252	66	0.91 (0.74, 1.13)
5–9	159	68	0.96 (0.76, 1.20)	36	17	1.58 (1.15, 2.19)	60	21	1.09 (0.77, 1.55)
≥10	419	175	0.94 (0.79, 1.14)	34	16	1.47 (1.02, 2.13)	49	23	1.25 (0.87, 1.80)
Frequency of use, per year
1	130	63	1.01 (0.81, 1.27)	231	69	0.96 (0.78, 1.19)	537	152	0.96 (0.83, 1.10)
2	141	58	0.94 (0.74, 1.19)	68	21	0.86 (0.58, 1.27)	126	38	1.01 (0.76, 1.34)
3–4	234	86	0.84 (0.67, 1.04)	21	11	1.62 (1.08, 2.42)	53	19	1.19 (0.80, 1.77)
≥5	233	98	0.97 (0.79, 1.19)	9	4	1.12 (0.51, 2.45)	32	21	1.39 (0.99, 1.95)
Duration (years) × frequency (per year)
<1 year × 1 per year	45	23	1.14 (0.84, 1.55)	116	34	0.96 (0.71, 1.29)	358	108	0.99 (0.84, 1.16)
1–4 years × 1 per year	43	17	0.88 (0.61, 1.28)	85	22	0.80 (0.54, 1.18)	144	32	0.82 (0.60, 1.10)
≥5 years × 1 per year	42	23	1.11 (0.83, 1.49)	30	13	1.48 (0.99, 2.21)	35	12	1.18 (0.74, 1.89)
<5 years × ≥2 per year	72	22	0.76 (0.54, 1.08)	58	16	0.71 (0.45, 1.13)	137	46	1.12 (0.88, 1.43)
≥5 years × ≥2 per year	536	220	0.94 (0.75, 1.17)	40	20	1.56 (1.14, 2.13)	74	32	1.16 (0.86, 1.56)

Adjusted for age in years (21–24, 25–29, 30–34, 35–39), year of enrollment (2014–2015, 2016–2020, 2021–2024), parent’s educational attainment (≤ high school, some college, ≥ college), history of anxiety or depression diagnosis (yes, no), participant’s educational attainment (≤ high school, some college, college, graduate school), and current smoking (yes, no).

Referent group for all comparisons.

Adjusted for age, year of enrollment, and parent’s educational attainment (≤ high school, some college, ≥ college).

CI, confidence interval; PR, prevalence ratio.

Associations of relaxer use with dysmenorrhea

Current (versus never) relaxer use was positively associated with dysmenorrhea (fully-adjusted PR = 1.30, 95% CI: 0.94, 1.80; Table 3). Several measures of lifetime relaxer use were positively associated with dysmenorrhea. In separate analyses of current and former (versus never) users, associations of measures of lifetime relaxer use with dysmenorrhea were generally stronger among current users (Supplementary Table S7). Nonetheless, we observed positive associations with dysmenorrhea prevalence in the highest exposure categories for measures of lifetime relaxer use among former users, relative to never use.

Across sensitivity analyses, current relaxer use was positively associated with dysmenorrhea (Supplementary Table S8). The association was partially attenuated after the exclusion of participants with a diagnosis of PCOS, endometriosis, or UL. Former (versus never) relaxer use was not meaningfully associated with dysmenorrhea prevalence in sensitivity analyses.

Current relaxer use was positively associated with dysmenorrhea among non-Black participants of minoritized racial and ethnic groups and, to a lesser extent, among non-Hispanic White participants, but not among Black participants (Table 5). Current relaxer use was associated with ∼30% higher dysmenorrhea prevalence across strata of age (Supplementary Table S9) and BMI (Supplementary Table S10).

Table 5.

Relaxer Use and Prevalence of Dysmenorrhea in Pregnancy Study Online, 2014–2024, Stratified by Race and Ethnicity

	Black			Non-Black minoritized racial and ethnic groups			Non-Hispanic white
	N	Cases	PR (95% CI)^a	N	Cases	PR (95% CI)^a	N	Cases	PR (95% CI)^a
Never use of relaxers^b	262	32	1.00 (ref.)	1,820	170	1.00 (ref.)	10,469	800	1.00 (ref.)
Former use	560	78	1.09 (0.74, 1.62)	277	21	0.96 (0.62, 1.48)	648	55	1.02 (0.78, 1.34)
Current use	178	27	1.06 (0.64, 1.78)	52	9	2.11 (1.14, 3.91)	100	10	1.23 (0.62, 2.41)
Age at first use, years
<10	242	37	1.21 (0.78, 1.90)^c	22	4	2.24 (0.96, 5.21)^c	46	4	1.38 (0.55, 3.45)^c
10–14	340	50	1.08 (0.70, 1.66)^c	74	9	1.36 (0.70, 2.66)^c	179	15	1.10 (0.68, 1.79)^c
15–19	115	13	0.87 (0.48, 1.59)^c	106	9	0.96 (0.51, 1.81)^c	240	20	0.90 (0.56, 1.44)^c
≥20	41	5	1.02 (0.43, 2.46)^c	127	8	0.83 (0.42, 1.65)^c	283	26	1.24 (0.82, 1.86)^c
No. of times burned during application
0	197	22	0.91 (0.55, 1.53)	272	15	0.74 (0.44, 1.24)	653	56	1.06 (0.81, 1.38)
1–4	322	48	1.08 (0.71, 1.66)	48	14	2.70 (1.68, 4.36)	81	9	0.98 (0.45, 2.11)
≥5	219	35	1.26 (0.80, 2.00)	9	1	—^d	14	0	—^d
Duration of use, years
<1	52	6	0.91 (0.40, 2.05)	127	11	1.12 (0.63, 2.01)	387	32	1.01 (0.72, 1.42)
1–4	108	16	1.19 (0.68, 2.08)	132	12	1.14 (0.67, 1.95)	252	20	1.00 (0.64, 1.55)
5–9	159	22	1.08 (0.66, 1.77)	36	2	0.76 (0.20, 2.93)	60	9	1.82 (0.99, 3.36)
≥10	419	61	1.08 (0.71, 1.66)	34	5	1.65 (0.67, 4.05)	49	4	0.55 (0.09, 3.38)
Frequency of use, per year
1	130	17	1.02 (0.59, 1.75)	231	20	1.15 (0.74, 1.80)	537	41	0.97 (0.71, 1.32)
2	141	22	1.22 (0.73, 2.03)	68	4	0.74 (0.29, 1.86)	126	14	1.28 (0.73, 2.25)
3–4	234	28	0.92 (0.56, 1.51)	21	4	2.03 (0.88, 4.69)	53	6	1.35 (0.59, 3.06)
≥5	233	38	1.22 (0.78, 1.92)	9	2	1.32 (0.20, 8.84)	32	4	1.04 (0.38, 2.89)
Duration (years) × frequency (per year)
<1 year × 1 per year	45	5	0.86 (0.37, 1.99)	116	10	1.13 (0.61, 2.08)	358	29	1.01 (0.71, 1.44)
1–4 years × 1 per year	43	5	0.88 (0.38, 2.03)	85	9	1.38 (0.74, 2.60)	144	9	0.87 (0.46, 1.65)
≥5 years × 1 per year	42	7	1.08 (0.55, 2.12)	30	1	—^e	35	3	0.89 (0.24, 3.35)
<5 years × ≥ 2 per year	72	12	1.21 (0.69, 2.12)	58	4	0.82 (0.34, 1.98)	137	14	1.12 (0.63, 1.97)
≥5 years × ≥ 2 per year	536	76	0.91 (0.56, 1.46)	40	6	1.20 (0.56, 2.56)	74	10	1.46 (0.75, 2.81)

Adjusted for age in years (21–24, 25–29, 30–34, 35–39), year of enrollment (2014–2015, 2016–2020, 2021–2024), parent’s educational attainment (≤ high school, some college, ≥college), history of anxiety or depression diagnosis (yes, no), participant’s educational attainment (≤ high school, some college, college, graduate school), and current smoking (yes, no).

Referent group for all comparisons.

Adjusted for age, year of enrollment, and parent’s educational attainment (≤high school, some college, ≥college).

Participants who reported being burned ≥5 times were grouped with those who reported being burned 1–4 times.

Participants with duration ≥5 years and frequency 1/year were grouped with participants with duration ≥5 years and frequency ≥2/year.

CI, confidence interval; PR, prevalence ratio.

Discussion

Among North American individuals who were trying to conceive, current (versus never) use of relaxers was associated with higher prevalence of AUB and dysmenorrhea, defined by self-reported menstrual cycle characteristics. Cycle irregularity was the most prevalent AUB symptom and had the strongest association with relaxer use.

Among current relaxer users, several measures of lifetime relaxer use were positively associated with AUB prevalence, relative to never use. Measures of lifetime use were not meaningfully associated with AUB prevalence among former users. In contrast, several measures of lifetime use were positively associated with dysmenorrhea prevalence among former, as well as current, users. Previously, we identified associations of current and former relaxer use, and some measures of lifetime use, with lower fecundability.⁶ Taken together, our results suggest the possibility of both short-term and long-term effects of relaxer use on ovarian and uterine function and identify a potential mechanism (i.e., irregular cycles) by which relaxer use may influence fertility.

To our knowledge, this is the first epidemiological study of personal use of relaxers and menstrual function during adulthood. Occupational studies have investigated the prevalence of menstrual disturbances among hairdressers compared with other workers. Most studies reported positive associations between hairdressing occupation and menstrual disturbances,^53–56 whereas another study reported no meaningful associations.⁵⁷ Given that hairdressers are exposed to multiple products,⁵⁸ these associations are not necessarily attributable to relaxers. Our findings are consistent with those of studies that observed associations of personal use of relaxers with several adverse reproductive health outcomes.^6,22–32

Relaxers are a source of exposure to EDCs and other toxicants. Some Keratin/Brazilian relaxers contain formaldehyde and other chemicals (e.g., ethylene glycol, siloxanes) that release formaldehyde gas when heated during application.^16,20,59,60 Other EDCs measured in relaxer products include bisphenol A and other alkylphenols, diethyl phthalate, and methyl paraben.¹⁵ The endocrine-disrupting functions of these chemicals include antiandrogenic (e.g., phthalates⁶¹ and parabens⁶²) and estrogenic (e.g., alkylphenols,^63,64 cyclosiloxanes,⁶⁵ phthalates,^61,66 and parabens⁶⁷) activity.⁶⁸

Chemicals found in relaxers have been linked to adverse reproductive outcomes. Researchers have observed reproductive toxicity of thioglycolate in laboratory studies,^56,69 and higher rates of menstrual disturbances among hairdressers occupationally exposed to thioglycolate compared with unexposed workers.⁵⁶ Formaldehyde is a known carcinogen and a suspected reproductive and developmental toxicant.⁷⁰ Exposure to formaldehyde is associated with a higher prevalence of menstrual disturbances⁷¹ and lower concentrations of antimüllerian hormone, a biomarker of functional ovarian reserve.⁷² In prospective studies, biomarkers of phthalates, phenols, and parabens are associated with reproductive hormone concentrations^73,74 and menstrual cycle characteristics.^75,76 These findings are consistent with in vitro and in vivo studies demonstrating reproductive toxicity of nonpersistent EDCs.^77–80

We observed that experiencing burns during the application of relaxers was associated with a higher prevalence of dysmenorrhea and, in some subgroups, AUB. Breakage of skin on the scalp may facilitate higher internal exposure to relaxer chemicals and chemicals from other hair products. Leave-in hair products (e.g., hair oils, leave-in conditioners) may not be washed out for days to weeks,^5,7,23,81 resulting in higher cumulative exposure from non-relaxer products that could be magnified if the scalp is damaged from relaxer use.²¹ Burns most frequently occur in the application of sodium hydroxide, lithium hydroxide, and potassium hydroxide relaxers.⁸² An experimental study using an in vitro epidermis model found that application of lye and no-lye relaxers was associated with >350% and 200% increased expression of prostaglandin E₂, respectively, compared with the control, providing a potential mechanistic link with dysmenorrhea.⁸³

We observed positive associations of relaxer use with menstrual disturbances among non-Black participants of minoritized racial and ethnic groups and non-Hispanic White participants. There were not enough participants within subgroups to disaggregate non-Black participants of minoritized racial and ethnic groups in analyses, and associations may have been obscured if products marketed to specific ethnic groups are particularly harmful. Unexpectedly, we did not observe meaningful associations of relaxer use with menstrual disturbances among Black participants. Our most plausible explanations relate to selection bias, discussed in the Limitations section.

We hypothesized that relaxer use would have a stronger association with menstrual disturbances among older participants because of greater susceptibility of older oocytes to environmental toxins. Contrary to this hypothesis, we observed that current (versus never) relaxer use and measures of lifetime use were positively associated with AUB prevalence among participants aged 21–29 years, but not among participants aged 30–39 years. Current (versus never) relaxer use was associated with a higher dysmenorrhea prevalence in both age groups. We observed a monotonic association between younger age at first use and higher prevalence of dysmenorrhea among participants aged 30–39 years, consistent with hypothesized long-term effects of exposure.

We hypothesized that associations would be stronger among participants with lower BMI due to the sequestering of lipophilic EDCs in adipose tissue. Indeed, we observed that current relaxer use and measures of lifetime relaxer use were positively associated with AUB prevalence among participants with BMI < 30 kg/m². Previous research also identified stronger positive associations of relaxer use with subfertility⁶ and incident UL²⁴ among participants with BMI <30 kg/m². Current relaxer use and measures of lifetime use were associated with higher dysmenorrhea prevalence in both BMI strata.

Limitations

Our study could be susceptible to reverse causation bias if participants reduced their use of relaxers in an effort to improve their reproductive health. We attempted to mitigate this bias through sensitivity analyses and found that the associations of current relaxer use with AUB and dysmenorrhea were similar after excluding participants with ≥12 cycles of pregnancy attempt time at enrollment. Furthermore, if participants stopped using relaxers because of menstrual disturbances, then we would expect to observe positive associations between former use and prevalence of menstrual disturbances. We did not observe such associations, providing evidence against reverse causation.

Selection bias could occur if relaxer use and menstrual disturbances jointly influenced participation in PRESTO. Menstrual disturbances may cause concern about fertility and thus increase individuals’ interest in participating in PRESTO. Relaxer use could also influence concern about fertility. We observed stronger associations of current relaxer use with AUB and dysmenorrhea among participants with ≤6 cycles of pregnancy attempt time and no history of infertility at study entry—a subgroup in whom concern about fertility is less likely to influence participation—suggesting that selection bias may distort the observed associations downward.

Media coverage of research on the potential health effects of relaxer use could influence participation and cause selection bias. Reassuringly, sensitivity analyses excluding participants who enrolled after October 2022 (i.e., publication date of the epidemiological study of relaxer use and uterine cancer that received extensive media coverage) yielded similar results to the main analysis, suggesting that media coverage did not meaningfully bias our observed measures of association.

Selection bias could have distorted associations differentially across racial and ethnic groups. The concept of pregnancy planning may have different interpretations across population subgroups.^84–88 Therefore, the factors influencing participation in a pregnancy planning cohort may vary according to sociodemographic factors. Black participants were more likely to report menstrual disturbances, a history of infertility, and longer pregnancy attempt times at enrollment than other racial and ethnic groups. While population-level racial disparities in infertility and gynecological health likely contributed to these observations,^89–93 these patterns may also suggest that concerns about one’s fertility disproportionately drove participation in PRESTO among eligible Black individuals. Consistent with our results, downward bias may be more pronounced among Black participants if use of relaxers and menstrual disturbances have a stronger joint influence on study participation in this group than the other groups. We lacked data with which to fully evaluate the direction or magnitude of such potential selection bias, overall or within subgroups. Importantly, the associations of current and former relaxer use with lower fecundability that we reported previously in a prospective analysis—a design less prone to selection bias—were observed in all racial and ethnic groups, including Black participants.⁶

Our exposure assessment was subject to several limitations. We did not collect detailed data on the timing of relaxer use, nor time since discontinuation among former users. The induction period for effects of relaxers on menstrual function is unknown, and analysis of exposure data from etiologically-irrelevant time periods may contribute to non-differential exposure misclassification. We had limited data on product brands and were unable to evaluate relaxer ingredients. As for outcome assessment, our measure of irregular cycles lacked specificity, because we did not provide the clinical definition of ≥8 to 10 days’ inter-cycle variation.³⁷ Our assessment of heavy menstrual bleeding did not include impact on quality of life nor did it account for product absorbency or saturation. Finally, there may be non-differential misclassification of exposures and outcomes due to imperfect recall.

Although we adjusted for several potential confounders, our results may be biased by residual confounding. We did not adjust for other sources of EDC exposure, such as other personal care products or diet. We did not collect data related to beliefs about racialized beauty norms, which influence behaviors related to product use² and could increase the risk of menstrual disturbances via psychological stress. We adjusted for history of anxiety or depression diagnosis, an imperfect proxy for psychological distress. Some associations were weak and could be explained by confounding. Small numbers could have led to chance findings, especially in stratified analyses. We conducted this research in a cohort of individuals who were trying to conceive, which may limit generalizability.

Strengths

Our study investigates a novel hypothesis. We analyzed a large cohort of menstruating individuals who were not using hormones. Participants provided detailed data on relevant measures. We build upon previous research on the health outcomes associated with relaxer use and provide insight into a potential mechanism by which relaxer use may harm fertility.

Conclusions

We observed positive associations of current use and measures of lifetime use of relaxers with the prevalence of menstrual disturbances in a cohort of >14,000 menstruating individuals. Our results may be biased downward, particularly within the subgroup of Black participants, among whom we did not observe meaningful associations of relaxer use with menstrual disturbances. The limitations of this work include potential for selection bias, residual confounding, and chance findings due to small numbers. We interpret our results as hypothesis-generating. Future research in prospective cohort studies could build upon this work by collecting and analyzing detailed data on relaxer use and menstrual cycle characteristics throughout the life course.

Authors’ Contributions

R.J.G.: Conceptualization, data curation, formal analysis, investigation, software, and writing—original draft. S.S.: Conceptualization, validation, and writing—review and editing. N.L.N.: Conceptualization and writing—review and editing. T.J.-T.: Conceptualization and writing—review and editing. D.D.B.: Conceptualization, supervision, and writing—review and editing. L.A.W.: Conceptualization, funding acquisition, investigation, project administration, resources, supervision, and writing—review and editing. A.K.W.: Conceptualization, investigation, supervision, and writing—review and editing.

Footnotes

Acknowledgments

The authors thank the PRESTO participants for their contributions to this study. The authors thank Michael Bairos for technical support in developing the Web-based infrastructure of PRESTO; Dmitrii Krivorotko, Krystal Kuan, and Tanran Wang for data management; and Alina Chaiyasarikul, Jessica Levinson, Martha Koenig, Andrea Kuriyama, and Eliza Pentz for general study support.

Author Disclosure Statement

Lauren Wise has received in-kind donations for primary data collection in PRESTO from and Swiss Precision Diagnostics. The other authors have no potential conflicts of interest to disclose.

Funding Information

This work was supported by grants R01-HD086742, R01-HD086742-S2, R01-ES029951, and R01-ES026166 from the National Institutes of Health.

Supplemental Material

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