Published: 21:39 BST, 21 October 2025 | Updated: 21:51 BST, 21 October 2025

Chemical relaxers used to straighten hair may increase the risk of developing multiple cancers by as much as 166 percent, a new study has found.

Experts have long been concerned about the presence of the carcinogen formaldehyde in chemical hair-straightening and relaxing treatments, such as Keratin treatments and Brazilian blowouts.

And in a bid to see the long term health consequences of this chemical, researchers from Emory University in Georgia analyzed data from a long-term health study involving more than 50,000 US women.

The Sister Study enrolled participants who were between 35 and 74 years old, had no personal history of breast cancer, but had at least one sister who had been diagnosed with the disease.

Enrollment took place between 2003 and 2009, and participants were followed through September 2021.

For the final analysis, researchers included data from 46,287 women who were followed for an average of 13.1 years.

The study focused on cancers with at least 100 reported cases during the follow-up period and examined whether participants had used hair straighteners or relaxers in the 12 months prior to joining the study.

Use of these products was linked to significantly higher cancer risks. Compared to non-users, women who used hair straighteners or relaxers had a 166 percent higher risk of pancreatic cancer, which is considered one of the deadliest forms of the disease because it lacks a specific screening test, making early detection difficult.

Chemical relaxers used to straighten hair may increase the risk of developing multiple cancers by as much as 166 percent, a new study has found