HAIR RELAXER GUIDE · RESEARCH
The Hair Relaxer Cancer Studies, Explained
Last updated: September 6, 2026
People looking for the hair relaxer study usually mean one paper: the 2022 NIH Sister Study analysis. It is the research the lawsuits are built on. Here is what it found, what came after it, and the one thing worth understanding about how it was built.
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 Sister Study analysis is the research behind the litigation.
- Frequent users had roughly double the uterine cancer rate of non-users.
- Double a small risk is still a small risk, and both framings matter.
- These studies watch what happens. They cannot prove what caused it.
The 2022 Sister Study
The Sister Study is run by an institute within the NIH and has followed tens of thousands of women for years, looking at what might be connected to cancer and other conditions. This analysis covered 33,497 women aged 35 to 74, followed for nearly 11 years, among whom 378 uterine cancers were diagnosed.
In October 2022 an analysis of that data was published in the Journal of the National Cancer Institute. Women who used chemical straighteners more than four times in the previous year developed uterine cancer at a higher rate than women who did not use them at all.
What the Numbers Meant
The headline was a roughly doubled risk. The part that usually gets left out is what it doubled from.
The researchers put it plainly. Of women who never used straighteners, an estimated 1.64% would develop uterine cancer by age 70. Among frequent users the estimate was 4.05%.
Reading those two numbers together is what keeps this in proportion. The jump is real and worth taking seriously. It is also a jump from about 2 women in 100 to about 4 in 100, which leaves the great majority of women in both groups unaffected.
Why How It Was Built Matters
The Sister Study watched. It did not assign anyone to use these products, because no one could ethically run that experiment.
That makes it good at spotting patterns across a lot of people and unable to prove what caused them. The women who used straighteners frequently differ from those who did not in other ways as well, and a study of this kind can only account for the differences it knows to measure. That limitation is built into the design, not a flaw in the work.
What Came After
The 2022 paper did not stand alone. Later work, including analyses from the Black Women’s Health Study, has looked at hair product use and reproductive health in the population that has used these products most. Earlier research had examined hair products and other hormone-related conditions.
Together it has built a case for a hormonal explanation while leaving the causation question open.
Why It Might Work That Way
Some products contain chemicals that interfere with hormones: certain phthalates, parabens, formaldehyde or preservatives that release it, and bisphenol A. The cancers under study are hormone-driven, which is why those chemicals became the focus. Scalp burns from relaxers may also let more of them into the body.
It is a plausible explanation. It has not been proven.
How the Research Reaches the Courtroom
The 2022 study is what the plaintiffs point to. Whether it convinces a court is what the litigation will decide. See Hair Relaxer Cancer Lawsuit and Hair Relaxer Lawsuit Updates. For fibroids, see Hair Relaxers and Fibroids.
Does the research describe your own history? Long-term relaxer use followed by a uterine, endometrial, or ovarian cancer diagnosis is the pattern these cases are built on. A free, no-obligation case review on Lawsuit Center is the way to find out whether yours fits.
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