HAIR RELAXER GUIDE · RESEARCH
The Hair Relaxer Cancer Studies, Explained
Last updated: June 9, 2026
People searching for "the hair relaxer study" are usually looking for the 2022 NIH Sister Study, which is the research that launched the litigation. This page summarizes what that study found, what later research has added, and the most important reading skill of all: understanding why an observational study can report an association without proving cause and effect.
For the bottom-line question, see Does Hair Relaxer Cause Cancer?
This page provides general educational information only and does not constitute medical or legal advice. It summarizes research at a high level and is not a substitute for the original studies or a conversation with a medical professional.
- The 2022 NIH Sister Study is the central research behind the hair relaxer litigation.
- It reported that frequent users of chemical straighteners had a higher rate of uterine cancer than non-users.
- Later studies, including a Black Women's Health Study analysis, have examined related questions.
- These are observational studies that report associations, not proof of causation.
The 2022 NIH Sister Study
The Sister Study is a large, long-running research project run by the National Institute of Environmental Health Sciences, part of the NIH, that has followed tens of thousands of women over many years to study factors related to cancer and other conditions. In October 2022, an analysis using this data was published in the Journal of the National Cancer Institute. It reported that women who used chemical hair straightening products frequently, defined as more than four times in the prior year, had a higher rate of uterine cancer than women who did not use them. This is the study most often described as the scientific foundation of the litigation.
What the Numbers Meant
The headline finding was a roughly doubled relative risk of uterine cancer among frequent users. Just as important is the absolute risk: the study estimated that lifetime uterine cancer risk rose from a low single-digit percentage among non-users to a somewhat higher single-digit percentage among frequent users. Both framings are true, and reading them together is what keeps the finding in perspective. A doubled risk of an uncommon outcome is still an uncommon outcome, while also being a real and meaningful signal worth studying.
Why the Design Matters
The Sister Study is observational, meaning it watched what happened among women who did and did not use these products rather than assigning use experimentally. Observational studies are powerful for detecting signals across large populations, but they compare groups that can differ in other ways, so they report associations rather than proving that one thing caused another. Good studies work hard to account for other factors, but the limitation is built into the design. This is the single most important idea for reading any headline about it.
Later and Related Research
The 2022 study did not stand alone. Subsequent research, including analyses from the Black Women's Health Study, has examined related questions about hair product use and reproductive health outcomes, reflecting that these products have been used predominantly by Black women over many years. Earlier work had also looked at hair products and other hormone-related conditions. Taken together, the body of research has built interest in a possible hormonal mechanism while leaving causation an open scientific question.
The Proposed Mechanism
Researchers have focused on endocrine-disrupting chemicals that can be present in some products, including certain phthalates, parabens, formaldehyde or formaldehyde-releasing preservatives, and bisphenol A. Because these can interfere with hormone pathways, and because the cancers studied are hormone-related, that is the leading proposed explanation under investigation. It is a hypothesis supported by biological plausibility, not a proven mechanism.
How the Research Connects to the Cases
The 2022 study is what plaintiffs' attorneys point to as the scientific basis for the hair relaxer lawsuits. How persuasive that evidence ultimately proves in court is something the litigation itself will test. For the legal overview and current status, see Hair Relaxer Cancer Lawsuit and Hair Relaxer Lawsuit Updates. For the related fibroid question, see Hair Relaxers and Fibroids.
