Hair Relaxers Tied to Higher Risk of Non-Reproductive Cancers, According to NIH Study

A recently published analysis from the Sister Study, a large US prospective cohort, provides new evidence linking the use of chemical hair straighteners or relaxers with the incidence of several cancers beyond those previously evaluated.¹ Prior research has shown associations between these products and hormonally driven malignancies, including breast, ovarian, and uterine cancers, but this study extends that scope to additional tumor types.²

The authors emphasize the public health relevance by noting that “approximately 2 million incident cancer cases are estimated to occur in the United States in 2025,” and that a substantial proportion of risk—“70–90% of lifetime cancer risk can be attributed to exogenous drivers such as environmental exposures”—may be influenced by consumer products, including chemical hair products. Because straighteners are widely used, often beginning early in life and disproportionately among Black women, understanding potential carcinogenic risks remains important.

Study Design and Population

The Sister Study enrolled 50,884 women aged 35 to 74 between 2003 and 2009, all of whom had a sister diagnosed with breast cancer but were cancer-free at enrollment. After exclusions, 46,287 participants were included. Data collection involved comprehensive baseline questionnaires, anthropometric assessment, biospecimen sampling, and long-term annual follow-up. Participants were asked about their use of straighteners in the prior 12 months, with exposure categorized as no use, ≤4 uses per year, or more than 4 uses per year.

Cancers included in this analysis were those with at least 100 incident cases, excluding breast, ovarian, and uterine cancers, which had been previously examined. Incident cancers were validated using pathology reports and national mortality data when available.

Key Findings

Participants were followed for an average of 13.1 years. The study identified incident cases of melanoma, thyroid, lung, non-Hodgkin’s lymphoma, leukemia, pancreatic, colorectal, and kidney cancers. Compared with non-users, women who used straighteners in the prior year showed significantly higher risks of several cancers. Pancreatic cancer demonstrated a hazard ratio of 2.66 (95% CI 1.25–5.66). Thyroid cancer risk was also elevated, with a hazard ratio of 1.71 (95% CI 1.01–2.89). Non-Hodgkin’s lymphoma showed a possible association, with a hazard ratio of 1.62 (95% CI 0.94–2.80), although confidence intervals included 1. Kidney cancer risk was likewise elevated but imprecise, with a hazard ratio of 1.60 (95% CI 0.76–3.36).

Higher frequency of use—defined as more than 4 times per year—was associated with increased incidence of pancreatic cancer, thyroid cancer, and non-Hodgkin’s lymphoma, with a significant dose-trend for pancreatic cancer. However, as the authors note, the precision of these estimates varied, and monotonic trends were not consistently observed across outcomes. Associations among non-Hispanic White women mirrored those in the overall population. Among Black women, estimates were largely inconclusive due to small case numbers, despite their substantially higher prevalence of hair straightener use.

Chemical Exposure and Biologic Rationale

Hair straighteners contain multiple compounds linked to carcinogenic or endocrine-disrupting mechanisms, including formaldehyde and formaldehyde-releasing agents, phthalates, parabens, and cyclosiloxanes. The study describes several biologically plausible pathways that support concern. Formaldehyde can form DNA adducts resulting in genomic instability, a hallmark of cancer. The application process can generate reactive oxygen species, contributing to oxidative damage in DNA, lipids, and proteins. Phthalates have been associated with metabolic dysfunction, inflammation, and endocrine disruption, processes linked to carcinogenesis. Parabens may alter thyroid signaling pathways, while certain ingredients, such as glyoxylic acid, have been implicated in potential kidney injury.

These mechanisms are relevant to cancers of diverse organ systems. This offers a scientific rationale for investigating a broad range of tumor types in relation to straightener use.

Strengths and Limitations

This study benefits from its large size, long duration of follow-up, prospective design, and detailed baseline data. Cancer outcomes were validated using medical records when possible, and analyses were adjusted for key confounders such as race/ethnicity, education, and smoking history.

However, several limitations warrant consideration. Exposure was measured only at baseline and may not reflect lifetime patterns, which are particularly relevant for carcinogenesis. Information on product formulations was not available, precluding differentiation between lye and no-lye products or identification of specific chemical exposures. The inclusion of “pressing products” in the questionnaire may have resulted in misclassification if participants reported device-based straightening without exposure to chemical agents.

Clinical and Public Health Implications

This study raises important considerations for clinicians, researchers, and policymakers. Chemical hair straighteners may contribute to increased risk of pancreatic and thyroid cancers and potentially non-Hodgkin’s lymphoma, in addition to the previously documented associations with breast, ovarian, and uterine cancers. Because product use often begins in childhood and is disproportionately common among Black women, these findings underscore the need for diverse research populations and improved exposure assessment.

Conclusion

This large US prospective study provides new evidence that hair straighteners and chemical relaxers may be associated with elevated incidence of pancreatic and thyroid cancers and possibly non-Hodgkin’s lymphoma. The authors conclude that these findings “expand the scope of the adverse health outcomes that have been associated with these products,” highlighting the need for additional research to identify specific chemical constituents and clarify causal mechanisms. Further investigation in more diverse populations with detailed product-use histories will be essential to guide future clinical recommendations and regulatory action.

References

1. Bailey JT, Chang CJ, Gaston SA, Jackson CL, Sandler DP, O'Brien KM, White AJ. Use of hair straighteners and chemical relaxers and incidence of non-reproductive cancers. J Natl Cancer Inst. 2025. doi: 10.1093/jnci/djaf280.
2. Chang CJ, O'Brien KM, Keil AP, et al. Use of straighteners and other hair products and incident uterine cancer. J Natl Cancer Inst. 2022;114(12):1636-1645. doi:10.1093/jnci/djac165