What the research suggests so far, with real numbers and honest limits.
6 min read
In an NIH study of 33,947 women, those who used chemical hair straighteners more than four times in the past year were more than twice as likely to be diagnosed with uterine cancer.
Women who used straightening products more than four times in the past year had an estimated 4.05% chance of uterine cancer by age 70, compared with 1.64% for women who never used them. The lead author noted that uterine cancer is relatively rare, so the overall risk stays low. The study was observational, so it shows an association, not proof that straighteners cause uterine cancer. This is the newest and least settled research in the guide, so here's exactly what's known and what isn't.
The Sister Study, led by the National Institute of Environmental Health Sciences, followed the women for nearly 11 years, during which 378 uterine cancers were diagnosed. Researchers tracked what women reported using and who was later diagnosed.
The study did not detect a statistically significant association for hair dyes, bleach, highlights or perms. About 60% of the users of straighteners in the study were Black women, and the researchers said the findings may be especially relevant to that group.
The study didn't record brands or ingredients, so it can't say which chemicals might be responsible. It was the first study of its kind, other research has been mixed (see "What other research shows" below), and more research is needed.
33,947
women followed for nearly 11 years
NIH Sister Study
4.05% vs 1.64%
estimated uterine cancer by age 70 in frequent users vs never users
NIH Sister Study
2.4 per 100
the modeled difference in risk by age 70 (4.05 minus 1.64), an estimate and not proof of cause
calculated from the study figures
Not significant
for hair dye, bleach, highlights or perms: no statistically significant association was detected in this study
NIH Sister Study
"More than twice as likely" is a relative risk. The absolute numbers are modeled estimates of 1.64% and 4.05%, a difference of about 2.4 women in every 100 by age 70. That's worth knowing about, and it's also why the lead researcher stressed that the absolute risk is still low. These are estimates of risk, not proof that straighteners caused those cases.
Always ask two questions of a headline like this: how big is the risk to begin with, and how many people were studied? Here, 378 cancers occurred in 33,947 women, which is enough to see a pattern but not enough to settle every question. Because the study tracked what women reported rather than assigning anyone to use or avoid the products, it can show an association but not prove a cause.
A 2023 study of 44,798 Black women, the Black Women's Health Study, did not find a significant overall link between heavy relaxer use (15 or more years and at least 5 times a year) and uterine cancer. Among postmenopausal women, moderate and heavy use were linked to a higher risk, and among premenopausal women there was no link.
A 2025 Sister Study analysis of 46,287 women looked at other cancers. It found higher rates of thyroid and pancreatic cancer among women who used straighteners or relaxers, and a possible increase for non-Hodgkin lymphoma that could be due to chance. It found little evidence that more frequent use meant more risk, and the authors said further research is needed to confirm the findings.
Know how often you use them
The association in the study was with frequent use (more than four times in the past year). Using them less often is a personal precaution, but this research can't show that it lowers your risk.
Talk with your stylist
Ask for the product name and ingredient information. Some straightening products contain formaldehyde or release it when heated.
Protect your scalp
Follow the product instructions, and stop using a product that burns or irritates your skin. The researchers noted that scalp burns and lesions may increase absorption of chemicals, but that is a proposed explanation, not something the study showed.
Mention it to your doctor
Especially if you have unusual vaginal bleeding, including any bleeding after menopause. Don't wait for a routine appointment. Your doctor can advise on your own risk.
Myth
Any hair color or treatment raises uterine cancer risk.
What research says
The study did not detect a statistically significant association for dyes, bleach, highlights or perms.
NIH Sister Study
Myth
Scientists know which chemical is responsible.
What research says
The study didn't record brands or ingredients. Researchers say parabens, bisphenol A, metals and formaldehyde could be contributing, but none has been confirmed.
NIH Sister Study
Myth
Twice as likely means a high chance.
What research says
The estimates were 1.64% vs 4.05% by age 70, and the lead author noted uterine cancer is relatively rare.
NIH Sister Study
Myth
The research proves straighteners cause uterine cancer.
What research says
It shows an association. The study was observational, and other research has been mixed, so more studies are needed.
NIH Sister Study; Black Women's Health Study
Check what applies to you
Do you use chemical straighteners, and how often did you use them in the past year?
If you straighten, decide how often is right for you
Use your count from the past year when you schedule your next treatment. Fewer treatments is a personal precaution, not a proven way to lower risk.
Ask your stylist one question
Ask what's in the product and whether there's a lower-frequency option for your hair.
Know the symptom
Unusual vaginal bleeding, especially after menopause, should be checked by a healthcare professional. Don't wait for a routine screening appointment.
- National Institutes of Health (NIEHS): Sister Study: chemical hair straightener use and uterine cancer (Oct 2022)
- Journal of the National Cancer Institute: Use of straighteners and other hair products and incident uterine cancer (2022)
- Environmental Research: Hair relaxer use and risk of uterine cancer in the Black Women's Health Study (2023)
- Sister Study, NIEHS: Use of hair straighteners and chemical relaxers and incidence of non-reproductive cancers (2025)
This lesson is optional. The research is newer and less settled than anything else in this guide.