The short answer
NCI reports that women who used any straightening products in the previous 12 months had 1.8 times the risk of developing uterine cancer in one large study, and that frequent hair straightener use has been linked with breast cancer. Some products contain formaldehyde, a known carcinogen. Which specific chemicals matter is still being worked out.
In a large cohort study reported by NCI, women who used any straightening products in the previous 12 months had 1.8 times the risk of uterine cancer.
One large prospective study found women using hair straighteners at least once per month were 1.3 times as likely to develop breast cancer.
Among Black women, heavy users of lye-containing relaxers showed higher risk of estrogen receptor-positive breast cancers.
Some straightening products contain formaldehyde, which is identified as a carcinogen.
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The whole article, with every detail and caution.
This page is for anyone who uses, or has used, chemical hair straighteners or relaxers. It explains what NCI reports about cancer risk and what the numbers mean.
This one is not a rumor
Plenty of viral claims about cosmetics fall apart when you check them against federal sources. This one does not fall apart. The National Cancer Institute describes real published studies that found higher cancer rates among women who used chemical hair straighteners.
Here is what those studies reported, in NCI's own terms.
Uterine cancer. In a recent cohort study, women who had used any straightening products in the previous 12 months had 1.8 times the risk of developing uterine cancer.
Breast cancer. One large prospective study found that women who used hair straighteners at least once per month were 1.3 times as likely to develop breast cancer as women who did not use them. Among Black women, heavy users of lye-containing relaxers showed higher risk for estrogen receptor-positive breast cancers.
What those numbers mean, and what they do not
A figure like 1.8 times sounds alarming until you ask: 1.8 times what? It compares how often a disease showed up in one group against another. Uterine cancer is not common, so raising an uncommon risk still leaves most women in the study group free of the disease.
It also does not tell you that the product caused the cancer in any individual. Observational studies find patterns. They cannot follow a single woman's illness back to a single bottle.
Both things are true: the association is documented, and it cannot explain one particular person's diagnosis.
Why nobody can name the culprit ingredient
Straighteners and relaxers are not one product. They are dozens of formulations with different chemistry, sold and reformulated over decades. NCI notes that some contain formaldehyde, which is identified as a carcinogen (something that can cause cancer). NCI also states plainly that because hair straighteners and relaxers contain varying compositions of chemicals, future studies will try to narrow down which chemicals have specific health impacts.
That is a scientist's way of saying: we see the pattern, we do not yet know which ingredient drives it.
If you have used these products for years
Understanding the science is not the same as accepting blame for it. The NCI findings describe patterns in groups, not a verdict on your choices.
Reasonable responses
This page does not tell you to stop using these products. These are general ideas some people consider once they know the findings, not NCI instructions:
- Stretching out the time between chemical treatments, given that the studies focused on frequent use.
- Asking a stylist what is in a formulation, particularly about formaldehyde.
- Making sure the salon is well ventilated during treatment.
Knowing the symptoms is worth more than the worry
Since the strongest finding involves uterine cancer, the practical thing to carry away is not fear of a bottle. It is knowing what to mention to your care team. The CDC says abnormal vaginal bleeding is a main warning sign of uterine cancer, such as bleeding after menopause or between periods. It says bleeding is never normal after periods stop. If you have bleeding that is not normal for you, the CDC advises seeing your doctor right away. That is true whatever you have used on your hair.
Take this article to your next appointment if it helps. A question asked out loud tends to shrink faster than one carried alone.
Sources
Common questions
Does using a relaxer mean I will get cancer?
No. These studies compare rates between groups of women. A higher rate in a group does not predict what happens to any one person, and most women in these studies did not develop these cancers.
Is occasional use different from monthly use?
The findings NCI describes centre on frequent use. The breast cancer figure came from women using straighteners at least once per month, and the uterine finding came from use in the previous 12 months.
Why is formaldehyde mentioned?
NCI notes that some straightening formulations contain formaldehyde, which is identified as a carcinogen. Products vary in composition, which is one reason researchers cannot yet name the responsible ingredient.
What about ordinary hair dye?
NCI reports that personal hair dye use is classified as not classifiable as to its carcinogenicity to humans.
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Plain-language explanation of the published sources cited on this page. AI-assisted, source-checked, not clinician-reviewed.
Sources last checked: 2026-10-02 what this meansLast updated: 2026-08-11Next planned review: 2027-08-11
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Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes, and this is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and then compared with the sources listed on it, looking for statements those sources do not back up. That check is not a sentence-by-sentence record and can miss errors. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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