Hair relaxers and leave-in conditioners and oils, commonly used by Black/African American women, may contain estrogens or estrogen-disrupting compounds. Thus, their use may contribute to breast cancer risk. Results of the few previous studies on this topic are inconsistent. We assessed the relation of hair relaxer and leave-in conditioner use to breast cancer incidence in the Black Women's Health Study, a nationwide prospective study of Black women. Among 50 543 women followed from 1997 to 2017, 2311 incident breast cancers occurred. Multivariable-adjusted hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated using Cox proportional hazards regression for breast cancer overall and by estrogen receptor (ER) status. For heavy use (≥15 years of use for ≥7 times/year) of hair relaxers relative to never/light use (<4 years, no more than 1-2 times/year), the multivariable HR for breast cancer overall was 1.13 (95%CI: 0.96-1.33). Duration, frequency, age at first use and number of scalp burns were not associated with overall breast cancer risk. For heavy use of hair relaxers containing lye, the corresponding HR for ER+ breast cancer was 1.32 (95% CI: 0.97, 1.80); there was no association for non-lye products. There was no association of conditioner use and breast cancer. Results of this study were largely null, but there was some evidence that heavy use of lye-containing hair relaxers may be associated with increased risk of ER+ breast cancer. Consistent results from several studies are needed before it can be concluded that use of certain hair relaxers impacts breast cancer development.
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