Published March 2026

The NIH Study That Changed Everything for Hair Relaxer Users

Before October 2022, most women who used chemical hair relaxers had no reason to think their beauty routine was connected to cancer. Then a major federal study changed the conversation — and eventually sparked the largest hair care product litigation in history.

The Sister Study: Background

The Sister Study is a long-term cohort research project run by the National Institute of Environmental Health Sciences (NIEHS), part of the National Institutes of Health. It launched in 2003 with a specific goal: to understand what environmental and genetic factors contribute to breast cancer risk. To do that, researchers enrolled sisters of women who had been diagnosed with breast cancer — a population genetically predisposed to elevated risk, making it easier to identify environmental factors that push the odds higher or lower.

The study enrolled over 50,000 women across the United States and Puerto Rico and collected detailed information about their health, lifestyle, and environmental exposures — including personal care product use. Women provided biological samples, completed extensive questionnaires, and were followed over time. By the time the 2022 hair product analysis was published, researchers had more than a decade of follow-up data on tens of thousands of participants.

The specific 2022 analysis, published in the Journal of the National Cancer Institute, examined the relationship between hair product use and uterine cancer in 33,497 women who provided baseline data on their personal care product habits.

What the Study Found

The headline finding: women who used hair relaxers or straighteners more than four times in the year preceding enrollment had a 2.55 times higher risk of developing uterine cancer compared to non-users. The results were statistically significant and persisted after controlling for other known risk factors.

To put that in concrete terms: uterine cancer affects roughly 2 to 3 percent of American women over their lifetime. More than doubling that risk — which is what the Sister Study's findings suggest for frequent hair relaxer users — is a substantial population-level harm. Given that an estimated 65 percent of Black women used hair relaxers in the years studied, the potential scale of impact is significant.

The study also found elevated risks for other outcomes: ovarian cancer, endometriosis, and uterine fibroids. These findings were less definitive than the uterine cancer result but consistent with what researchers expected given the endocrine-disrupting chemicals found in hair relaxer formulations.

Why the Study Is Credible

Scientific findings only matter if the underlying research is well-designed. The Sister Study has several strengths that make its findings particularly credible.

Sample size: More than 33,000 women in the hair product analysis. This is a large enough sample to detect meaningful statistical differences even in relatively rare outcomes like specific cancer types.

Prospective design: Participants reported their hair product use before developing cancer — not after. This is important because retrospective studies (where you ask sick people what products they used in the past) can be subject to recall bias. In a prospective cohort, you know what people were doing before disease developed.

Long follow-up: The data spans more than a decade of follow-up. Short follow-up periods can miss cancers with long latency periods. Uterine cancer takes years to develop from initial cellular changes, so a long follow-up is essential for detecting a signal.

Established research institution: The NIEHS is one of the most respected environmental health research organizations in the world. This wasn't a study funded by a plaintiff's attorney or a competitor to the hair care industry. It was government-funded academic research from a federal health agency.

The Industry's Response

Hair care companies and their trade organizations pushed back on the Sister Study. They argued that the study relied on self-reported data, that it couldn't prove causation (only correlation), and that it hadn't identified which specific chemicals in relaxers were responsible for the elevated risk.

These are standard industry talking points in product liability disputes. They're not wrong that the study doesn't definitively prove that any specific chemical in any specific product caused any individual woman's cancer. But that level of certainty is not the legal standard for product liability, and it's not the scientific standard for public health action either.

Epidemiological studies show associations, and those associations — when large, statistically significant, biologically plausible, and consistent with other evidence — inform both regulatory action and legal liability. The hair relaxer litigation isn't claiming that this one study proves everything. It's citing this study as part of a larger evidentiary picture that includes chemical analysis, animal studies, and other epidemiological evidence.

What It Means Legally

The Sister Study is central to the hair relaxer litigation for one specific reason: it's the clearest evidence of when the connection between hair relaxers and uterine cancer became publicly known. For discovery rule purposes in product liability claims, the clock may start from when you knew or reasonably should have known about the connection. For many women, that was October 2022.

If you used hair relaxers for years and were diagnosed with uterine cancer before 2022, you may still have a viable claim — depending on your state's statute of limitations rules and when you actually connected your diagnosis to your hair care product use. An attorney familiar with hair relaxer litigation can help you work through that timeline.