Can Hair Relaxers Cause Fibroids?

Two different claims get welded together in almost every article on this subject. One is that relaxers cause uterine fibroids, which the primary research does not support. The other is that hair straighteners are associated with uterine cancer, which one large NIEHS cohort did find, at a hazard ratio of 1.80 for recent use and 2.55 for frequent use. Fibroids and uterine cancer are not the same condition and the evidence for each is in a completely different state. The FDA, contrary to what you will read almost everywhere, has not banned formaldehyde in hair straighteners.

Do hair relaxers cause fibroids?

The evidence says no. The NIEHS analysis of 4,162 Black women tested this directly and found nothing that clears the bar. Straightener use at ages 10 to 13 gave an odds ratio of 1.15, with a 95 percent confidence interval of 0.96 to 1.36. Frequent use gave 1.18, interval 0.99 to 1.42. Incident fibroids over follow-up gave a hazard ratio of 1.14, interval 0.81 to 1.63.

Every one of those intervals crosses 1. That is the entire finding, and it is the part that gets left out of the headlines.

The detail that settles the question is what happened in the comparison group. The same analysis found a similar association in White women, odds ratio 1.23 with an interval of 1.03 to 1.47, in a group where straightener use ran at 5 percent or less. If a chemical in the product were doing the damage, the association should track the exposure. It does not. It shows up just as strongly, slightly more strongly in fact, among women who mostly are not using the product. An association that survives the near-total removal of the exposure is not being produced by the exposure.

What does a confidence interval crossing 1 actually mean?

A confidence interval that crosses 1 means the data are compatible with the product raising risk, lowering risk, and doing nothing at all, so the study has not distinguished between those possibilities. That is the plain-English version, and most readers have never been given it.

An odds ratio of 1.15 sounds like a 15 percent increase. It is better understood as the middle of a range. When that range runs from 0.96 to 1.36, the honest reading is that the true effect could be a small decrease, no effect, or a moderate increase, and this study cannot tell you which. Reporting the 1.15 and dropping the interval converts an inconclusive result into a scary one, which is exactly what the content farms ranking for this query have done.

This is the difference between a finding and a rumor. When a risk figure is quoted without its interval, the interval is the first thing to go looking for. We describe how we check figures like these in how we research.

What about uterine cancer, which is a different question?

That one has a real signal, and it deserves to be stated as clearly as the fibroid null. The Sister Study followed 33,947 women aged 35 to 74 for about 10.9 years and recorded 378 uterine cancers. Women who used straighteners in the prior 12 months had a hazard ratio of 1.80, 95 percent confidence interval 1.12 to 2.88. Frequent use, meaning more than four times a year, gave 2.55, interval 1.46 to 4.45, with a P-trend of .002.

Those intervals do not cross 1. There is a dose-response pattern. This is a materially different quality of evidence from the fibroid analysis, and anyone who tells you the whole relaxer story is a myth is overcorrecting.

Note also what the study did not find. Hair dyes, bleach, highlights and permanent waves showed no association with uterine cancer. Whatever is going on is specific to straightening products, not to chemical hair processing in general.

How large is that uterine cancer risk in real numbers?

Small. A hazard ratio of 2.55 on a rare outcome is still a rare outcome. The NIH estimated that 1.64 percent of women who never used straighteners would develop uterine cancer by age 70, against 4.05 percent of frequent users.

Put in the form clinicians use, the number needed to harm is 85 for ever-use and 42 for frequent use. That means roughly 85 women would have to use straighteners for one additional uterine cancer to appear, and about 42 in the frequent-use group.

Both framings are true at once. The relative risk more than doubles. The absolute risk moves from about one and a half in a hundred to about four in a hundred. You are entitled to see both before deciding which one alarms you, and almost nobody publishing on this topic shows the second.

What does the uterine cancer study not show?

Quite a lot, and the authors say so themselves. It collected no brand or ingredient data. It identified no causal chemical. It found no difference in hazard ratios between racial or ethnic groups, which matters because this is routinely reported as a disparity finding when it is not one. Black women in the cohort used these products far more, so more of the burden falls on them, but the per-use risk is the same for everyone.

This is observational research. It cannot establish cause, and the word cause should not appear next to it.

Then there is a check that points the other way. A newer NIEHS occupational analysis looked at hairdressers, a group with heavy, sustained, professional exposure to these products, and found no association with uterine cancer, hazard ratio 1.04 with a 95 percent confidence interval of 0.60 to 1.77. If the products were straightforwardly carcinogenic, that is the population where you would most expect to see it.

Has the FDA banned formaldehyde in hair straighteners?

No, and this is the single most commonly published falsehood in this niche. Regulatory identification number 0910-AI83 sits at Proposed Rule Stage in the Unified Agenda. No proposed rule has ever been published. There is no Federal Register citation. The date for issuing a notice of proposed rulemaking has slipped six times.

The correct wording is that a rule is proposed for future rulemaking and has not yet been issued. Not banned. Not restricted. Not in effect.

This matters practically. Articles claiming the ban happened imply that products on the shelf have already been cleaned up by regulation. They have not been.

If relaxers are not driving fibroid risk, what is?

Something is, and it would be a poor correction that left you thinking nothing is happening. Stewart and colleagues reviewed 60 publications and reported that Black race was the only factor recurrently found to raise fibroid risk, at two to three times the rate in White women. That is the strongest and most consistent signal in the entire fibroid literature.

It also starts young. Marsh and colleagues scanned asymptomatic women aged 18 to 30 who had never been pregnant and found fibroids in 26 percent of the Black participants against 7 percent of the White participants. These were women with no symptoms and no diagnosis. The condition was already there.

So the risk is real, it is large, and it is not explained by a bottle of relaxer. The full picture, including what symptoms actually feel like and where treatment access breaks down, is in the guide to uterine fibroids in Black women. If your interest in this question started with your scalp rather than your uterus, the useful pages are the ones on hair loss and thinning edges and on telling dandruff from seborrheic dermatitis on natural hair.

Common questions about hair relaxers and fibroids

Do hair relaxers cause fibroids?

No. The NIEHS analysis of 4,162 Black women found odds ratios of 1.15 and 1.18 and a hazard ratio of 1.14, all with confidence intervals crossing 1. A similar association appeared in White women despite very low straightener use.

Is the uterine cancer link real?

The association is real and statistically significant. The Sister Study found a hazard ratio of 1.80 for recent use and 2.55 for frequent use. It is observational, identified no causal chemical, and the absolute risk moves from 1.64 percent to 4.05 percent by age 70.

Did the FDA ban formaldehyde in hair straighteners?

No. Regulatory identification number 0910-AI83 remains at Proposed Rule Stage with no proposed rule published, no Federal Register citation, and six date slips. The correct description is proposed for future rulemaking, not yet issued.

Why do fibroids affect Black women so much more?

The disparity is well documented and unexplained by hair products. A review of 60 publications found Black race the only factor recurrently reported to raise fibroid risk, at two to three times. Fibroids appear in 26 percent of asymptomatic Black women aged 18 to 30.

Sources

• NIEHS Sister Study, hair straightener use and uterine fibroids in 4,162 Black women. Published 2025. Checked when this page was last reviewed.
• Chang et al., Journal of the National Cancer Institute, hair product use and uterine cancer risk in the Sister Study. Cohort enrolled 2003 to 2009, published 2022. Checked when this page was last reviewed.
• National Institutes of Health, news release on hair straightening chemicals and uterine cancer risk, absolute risk estimates. Data 2003 to 2019. Checked when this page was last reviewed.
• NIEHS occupational analysis, hairdressing and uterine cancer. Hazard ratio 1.04, 95 percent CI 0.60 to 1.77. Checked when this page was last reviewed.
• OMB Office of Information and Regulatory Affairs, Unified Agenda entry RIN 0910-AI83, formaldehyde in hair smoothing products. Retrieved 2026. Checked when this page was last reviewed.
• Stewart et al., BJOG, systematic review of uterine fibroid epidemiology across 60 publications. Published 2017. Checked when this page was last reviewed.
• Marsh et al., Fertility and Sterility, ultrasound-detected fibroids in asymptomatic nulliparous women aged 18 to 30. Published 2013. Checked when this page was last reviewed.