Does Hair Relaxer Cause Cancer?

What the 2022 NIH Study Found

In October 2022, researchers published a study in the Journal of the National Cancer Institute using data from the NIH Sister Study, which followed tens of thousands of women over roughly a decade. They 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. The reported increase was meaningful in relative terms, but it is important to read it in context, which the next sections do. For a fuller look at this and related research, see The Hair Relaxer Cancer Studies, Explained.

"Associated With" Is Not "Caused By"

This distinction is the heart of the matter. An association means a condition appeared more often in a more-exposed group; it does not establish that the exposure caused any particular person's illness. Observational studies like the Sister Study are valuable for detecting signals, but they compare groups that can differ in many ways, so they cannot by themselves prove cause and effect. In litigation, whether a product can be tied to a specific diagnosis is a separate, fact-intensive question.

Putting the Numbers in Perspective

Uterine cancer is relatively uncommon over a lifetime, so even a doubled relative risk applies to a small baseline. The often-cited figures describe an estimated lifetime risk rising from a low single-digit percentage in non-users to a somewhat higher single-digit percentage in frequent users. Relative risk and absolute risk are different things, and understanding both is part of reading any health headline sensibly.

Which Cancers Are Discussed

The strongest and most-studied reported association is with uterine cancer, particularly endometrial cancer, which is the most common form. Some research has also examined ovarian and breast cancer. Uterine cancer is also the core of the federal litigation. A related but distinct condition, uterine fibroids, is covered separately in Hair Relaxers and Fibroids.

The Proposed Mechanism

Researchers have focused on endocrine-disrupting chemicals that can be present in some hair products, including certain phthalates, parabens, formaldehyde or formaldehyde-releasing preservatives, and bisphenol A. These compounds can interfere with hormone pathways, and because many of the cancers studied are hormone-related, that is one proposed biological explanation under investigation. It remains an area of active research rather than settled fact.

How This Connects to the Litigation

The 2022 study is widely described as the scientific foundation for the hair relaxer lawsuits, which allege that manufacturers failed to warn consumers about potential risks. For the legal overview, including the consolidated federal proceeding and who may be affected, see Hair Relaxer Cancer Lawsuit and the ongoing Hair Relaxer Lawsuit Updates.

What This Means for You

If you used chemical hair relaxers and have health concerns, the most reliable step is to talk with a qualified medical professional who knows your history. This page is meant to help you understand the research landscape, not to answer a personal medical question or to suggest that any individual's diagnosis was caused by a product.