Home Chemical Hair Straighteners

Chemical Hair Straighteners

Chemical hair straighteners break proteins in curly hair to straighten it permanently. Studies have linked these hair relaxers to an increased risk of certain hormone-related cancers, such as uterine and endometrial cancer. Black women are heavy users of these products and may be most at risk.

Last Modified: March 18, 2024
Fact Checked

Editors carefully fact-check all Drugwatch.com content for accuracy and quality.

Drugwatch.com has a stringent fact-checking process. It starts with our strict sourcing guidelines.

We only gather information from credible sources. This includes peer-reviewed medical journals, reputable media outlets, government reports, court records and interviews with qualified experts.

Why Trust DrugWatch?

Drugwatch.com has been empowering patients for more than a decade

Drugwatch.com has provided reliable, trusted information about medications, medical devices and general health since 2008. We’ve also connected thousands of people injured by drugs and medical devices with top-ranked national law firms to take action against negligent corporations.

Our team includes experienced medical writers, award-winning journalists, researchers and certified medical and legal experts. Drugwatch.com is HONCode (Health On the Net Foundation) certified. This means the high-quality information we provide comes from credible sources, such as peer-reviewed medical journals and expert interviews.

The information on Drugwatch.com has been medically and legally reviewed by more than 30 expert contributors, including doctors, pharmacists, lawyers, patient advocates and other health care professionals. Our writers are members of professional associations, including American Medical Writers Association, American Bar Association, The Alliance of Professional Health Advocates and International Society for Medical Publication Professionals.

About Drugwatch.com

  • Assisting patients and their families since 2008.
  • Helped more than 12,000 people find legal help.
  • A+ rating from the Better Business Bureau.
  • 5-star reviewed medical and legal information site.
Learn More About Us


"Drugwatch opened my eyes to the realities of big pharmacy. Having a family member with major depression and anxiety, I was looking for information on her medications. I found information that was very helpful, that her psychiatrist never told her."
Marianne Zahren Patient’s Family Member
  • Google Business Rating
  • BBB A+ Rating Logo

How Chemical Hair Straighteners Work

Chemical hair straighteners, also called relaxers, work by breaking up the natural chemical bonds that form and hold human curls. Chemical hair straighteners are widely popular because they’re effective at turning curly hair into straight hair. It’s a permanent hair straightening method that lasts until new hair grows out.

A protein called keratin makes up the structure of hair strands. Keratin has molecules known as sulfides. Sulfides can pair together to create a disulfide bond. As sulfide molecules reach toward each other to form disulfide bonds, the hair bends. The more the disulfide bonds in a strand of hair, the more it coils and curls.

Different brands of straighteners can affect hair differently. For example, some brands might dry hair out more, making it prone to breakage, according to professional hair stylist Tonya Herrell.

“I used to use this brand called Affirm, and I noticed that it made hair super dry. Clients might not notice, but I did because I know healthy hair. It just wasn’t giving it any life. I switched to a brand called Congo. It’s Black-owned and Black-created, and it keeps the hair lively and shiny,” Herrell told Drugwatch.

Chemicals in Hair Relaxers

Chemical hair straightening products use various chemicals to dissolve disulfide bonds, altering the protein structure’s shape in the hair and straightening it. Some of these chemicals, such as formaldehyde, can be toxic to humans.

Many chemical hair straighteners have proprietary formulas, so it’s not easy to see all the ingredients in each formula.

Chemicals in hair straighteners include:
  • Ammonium Thioglycolate
  • Benzophenone-3
  • Cyclosiloxanes
  • Diethanolamine
  • Formaldehyde
  • Parabens
  • Phthalates
  • Sodium Hydroxide (Lye)
  • Sodium Thioglycolate
  • Triclosan

If used incorrectly or too often, these chemicals can cause long-term damage to hair and skin and increase the risks for other health issues, including cancer.

Is Chemical Hair Straightening Safe?

In the short term, chemical hair straighteners don’t cause many serious side effects. However, heavy, long-term use comes with the potential for unsafe toxicity and an increased risk of hormone-related cancers. According to some recent studies, Black women — who are more likely to use these products long term — are at the most risk.

Side effects of chemical hair straighteners affect not only people who receive hair treatments but also the salon staff who apply them. For instance, formaldehyde is a known carcinogen. It can lead to significant breathing and lung problems.

Unfortunately, even products labeled formaldehyde-free may contain chemicals that release formaldehyde when subjected to heat.

Side Effects of Chemical Hair Straighteners
  • Allergic reactions
  • Alopecia
  • Atrophied skin
  • Burned scalp and skin
  • Damage to the hair shaft
  • Discolored hair
  • Eczema
  • Frizzy hair
  • Inflamed scalp
  • Irritated and painful scalp
  • Loss of hair

Parabens are chemicals routinely used in cosmetic hair care and skin care products. Shampoos, conditioners and other hair products often contain parabens because of their effectiveness against mold and fungi.

However, excessive paraben absorption can interfere with hormone production and cause hormone dysfunction, affecting male and female reproduction. Girls going through puberty are particularly vulnerable to these effects.

Recent studies have linked chemical hair straighteners to cancers such as uterine cancer, ovarian cancer and breast cancer. Some links, such as the link to breast cancer, have had mixed results in studies.

Uterine cancer is the top cancer linked to hair relaxers. According to researchers, parabens, phthalates, formaldehyde and the toxic chemical Di(2-ethylhexyl) phthalate, or DEHP, are among the ingredients in relaxers that may cause cancer.

Researchers theorize that these chemicals affect hormones that could contribute to cancer development.

Daniel A. Landau, M.D.
“It is hypothesized that some of the chemicals found in these products (such as phthalates, parabens and cyclosiloxanes) can essentially behave like hormones in the body [such as estrogen]. Elevated estrogen seems to increase the likelihood of development of uterine cancers.”

It’s important to note that the risk of uterine cancer, also referred to as endometrial cancer, may also increase with other factors that interfere with hormone balance. One of the biggest risks is excess body weight, according to oncologist Dr. Joseph Buscema of Arizona Oncology.

“The relationship between the increase in body weight and endometrial malignancy has been associated with the presence of excessive estrogen quantities in these individuals.  This factors into the development of endometrial cancer in younger cohorts of patients who have not gone through menopause, because of excess body weight, disturbance of ovulation that is associated with progesterone production to offset the proliferating influence of estrogen,” said Dr. Buscema.

Avoiding substantial weight gain and regular exercise are two ways women can reduce their uterine cancer risk, Dr. Buscema adds.

What Studies Say About Hair Relaxer Cancer Risk

A few recent studies have linked chemical straighteners to cancer. Some evidence presented is stronger than others, while other studies found a weaker link to breast cancer.

  • 2023:
    A study published in Environmental Research found that postmenopausal Black women who said they used hair relaxers more than twice a year for more than five years had at least a 50% increased risk of uterine cancer.
  • 2022:
    A National Institutes of Health study called the Sister Study found that the risk of uterine cancer was twice as high in women who used chemical straighteners more than four times a year compared to those who didn’t use them.
  • 2021:
    A study published in Carcinogenesis found that there wasn’t a strong link between hair relaxers and breast cancer risk in Black women. But women who used lye-containing straighteners at least eight times a year for 15 years or more had a higher risk of breast cancer.

In addition to cancer, researchers have also linked hair relaxers to other women’s reproductive health issues such as uterine fibroids and endometriosis.

Lawsuit Information
Women who used chemical hair straighteners and were diagnosed with uterine, endometrial and ovarian cancer are filing lawsuits.
View Lawsuits

Black Women and Chemical Hair Straightener Use

Chemical hair straightener companies have heavily marketed to Black women for years, and the use of these products is common in the community. In fact, according to a December 2023 study published in Environmental Research, up to 95% of self-identified Black women in the U.S. reported using chemical hair straighteners, or hair relaxers.

Researchers argue that heavy chemical straightener use among Black women has also increased their rates of uterine cancer. Kimberly Bertrand, associate professor of medicine at Boston University Chobanian & Avedisian School of Medicine, is one of the researchers who published the 2023 Environmental Research study.

Did you know?
Up to 95% of self-identified Black women in the U.S. have used chemical hair straighteners.

“Compared to non-Hispanic white women, Black women have higher rates of aggressive subtypes of uterine cancer and are nearly twice as likely to die from their disease,” Bertrand said.

Many Black women who filed chemical hair straightener lawsuits after being diagnosed with cancer claim they were never warned of the risk by hair relaxer makers and that the companies targeted Black women. As of June 2024, 8,170 lawsuits were pending in Illinois federal court.

FDA Proposed Ban on Formaldehyde in Hair Relaxers

In Spring 2023, the FDA announced it would ban formaldehyde and formaldehyde-releasing chemicals in chemical hair straighteners because of the serious health risks these chemicals pose. The proposed ban is supposed to take effect in April 2024.

“Use of hair smoothing products containing FA [formaldehyde] and FA-releasing chemicals is linked to short-term adverse health effects, such as sensitization reactions and breathing problems, and long-term adverse health effects, including an increased risk of certain cancers,” the Agency said.

Some consumer advocates say that it has taken far too long for the Agency to act.

“We’re glad that the FDA finally decided to ban these products, which the Agency started planning to do in 2016. The research evidence has been clear for more than a decade,” Dr. Diana Zuckerman, president of the National Center for Health Research, told Drugwatch. ”But we have to wonder how many women have been harmed in the meantime, and why did it take the Agency so long to finally do the right thing to protect consumers?”

Please seek the advice of a medical professional before making health care decisions.