Using chemical hair straighteners may increase the risk of uterine cancer, fibroids, breast cancer and endometriosis, according to studies. Side effects of frequent chemical hair straightener use include weakening of hair, increased frizz, hair discoloration and scalp inflammation.
Chemical hair straighteners are lotions or creams that break down the protein structure of curly hair to straighten it permanently. Chemically straightened hair typically stays straight for roughly six months.
Professional stylists administer most chemical hair straighteners at salons because of the potentially harmful chemicals involved. A few kits allow people to chemically straighten their hair at home, but they typically have weaker formulas and might be less effective. Chemical hair straighteners contain harsh chemicals that can damage hair over time and irritate the scalp.
According to California’s Department of Toxic Substances Control, hair straighteners contain chemicals like cyclosiloxanes, formaldehyde, parabens, diethanolamine, sodium hydroxide, phthalates, triclosan and benzophenone-3. Some of these substances could harm your health and the environment. According to the Department, they may cause breathing problems, harm development and reproduction, affect hormones and cause skin reactions. In addition, they can stick around in the environment for long periods and may build up in living things.
The U.S. Food and Drug Administration (FDA) has looked at banning formaldehyde in hair relaxers since at least 2016. That’s when the agency began a series of focus groups with salon owners, stylists and consumers about formaldehyde in hair products.
The FDA published a proposed federal rule banning formaldehyde from hair straighteners in spring 2023. The agency then missed the April 2024 deadline it set for itself to put the ban into effect.
Use of hair smoothing products containing [formaldehyde] 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.
In September 2024, the FDA missed another deadline for banning formaldehyde in hair relaxers. The FDA’s latest deadline is March 2025. These delays have left consumers and health advocates concerned about the continued availability of potentially harmful products on the market.
Types of chemical hair straighteners — sometimes called relaxers — include traditional chemical straightening, Japanese straightening and Brazilian keratin treatments.
Each type of straightening works for different levels of curly hair, from very curly to slightly wavy or frizzy. Stronger chemical treatments work for very curly hair and last longer, but they are more damaging to the hair and scalp.
Experts further classify chemical hair straighteners as lye relaxers, no-lye relaxers and thio relaxers, depending on the product’s ingredients. Permanent lye relaxers have the harshest chemicals and potentially damage hair most long-term.
Chemical hair straighteners break protein bonds in the hair known as disulfides. Curlier hair has more disulfide bonds. Stronger chemicals are often necessary to break these bonds in curlier hair.
After breaking the bonds, the stylist will apply a neutralizer to reform the disulfide bonds. While reforming the bonds, the stylist will configure the hair into its new shape to straighten it.
Gentler chemical hair straighteners use weaker chemicals to straighten hair, but they don’t last as long and won’t work on very curly hair. Some types of straighteners, such as Japanese or keratin straightening, don’t allow you to wet or wash your hair for at least 48 hours afterward.
The safest way to use chemical straighteners is to have a professional stylist apply them for you. Stylists receive training to properly apply chemicals that can damage your scalp and skin.
“Chemical hair relaxers are corrosive products that are meant to break down the hair protein so that the hair can be smoothed into a permanently straight position,” Ghanima Abdullah, a cosmetologist and beauty consultant at therighthairstyles.com, told Drugwatch. “The same corrosion can affect the hair follicles by breaking down the structures inside the hair follicle, leading to poor hair health and stunted growth.”
If you use a home kit to straighten your hair, follow all instructions and precautions. Set timers to avoid over-processing your hair. You can use petroleum jelly or base oils on your scalp and hairline to prevent skin irritation.
Chemically straightening your hair can cause long-term damage and scalp irritation. Over time, reapplying chemicals over previously straightened hair causes it to weaken and break.
Chemical hair straightener side effects include short-term problems such as scalp irritation and hair damage, as well as long-term side effects such as an increased risk of endometriosis. Studies have also linked hair straightening products to cancer.
“Recent studies have suggested a possible association between frequent use of chemical hair relaxers and an increased risk of hormone-related cancers, such as breast and uterine cancer,” Dr. Janet Vafae, a board-certified dermatologist in Los Angeles, told Drugwatch. “For instance, a study published in the International Journal of Cancer indicated a higher incidence of breast cancer among women who regularly used hair relaxers.” That 2019 study found that nearly 75% of Black women had used hair straighteners in the previous year.
A study from Boston University’s Black Women’s Health Study released in December 2023 found that postmenopausal Black women who used hair relaxers more than twice a year or for over five years had a more than 50% higher risk of uterine cancer compared to those who rarely or never used them.
A 2022 National Institute of Health study found women who used chemical hair straighteners had a significantly increased risk of uterine cancer compared to women who didn’t use them. Another NIH study in 2019 found women who used chemical hair straighteners had a 30% increase in the incidence of breast cancer. Black women are particularly at risk because they use hair straighteners at higher rates than other consumers.
Women who used straighteners and later developed uterine, endometrial and ovarian cancer filed chemical hair straightener lawsuits naming L’Oreal, Dark & Lovely, Namaste and other brands and parent companies as defendants. Plaintiffs said the defendants sold these harmful products even if they knew they could cause cancer and failed to warn the public.
A popular alternative to chemical hair straightening is heat styling with hair serums made without harsh chemicals. These methods last only a day or two but are safer than chemical straightening.
“We emphasize alternative methods for hair straightening that promote hair health, such as thermal styling and embracing natural textures,” Rachel Voss, a hairstylist and owner of Voss Salon outlets in the Dallas, Texas area, told Drugwatch. “These techniques avoid harsh chemicals and support long-term scalp and hair wellness.”
Heat styling can also damage hair but doesn’t carry a risk of cancer or other reproductive system issues. To reduce damage from heat styling, use a heat protectant on your hair, use the right temperature for your hair type and condition the hair after styling.
Several online recipes for natural hair straighteners made with coconut milk, fruits or oils promise effective straightening. However, these methods might not be reliable.
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