What Is DMDM Hydantoin?
DMDM hydantoin is a preservative that releases formaldehyde. The FDA lists it on its allergens in cosmetics page under formaldehyde and formaldehyde releasing ingredients, alongside bronopol and diazolidinyl urea. The European Commission explains that such preservatives gradually release formaldehyde to preserve the finished product, and that they are used in both leave-on and rinse-off cosmetics.
On its page about hair smoothing products, the FDA says skin sensitization or allergic dermatitis can develop after repeated contact with formaldehyde-related ingredients, and that formaldehyde released into the air can seriously irritate the eyes, nose and lungs. Under Regulation (EU) 2022/1181, finished products containing listed formaldehyde releasers must carry the warning "releases formaldehyde" when total formaldehyde released exceeds 0.001 percent (10 ppm), and products that met the earlier rules could stay on the EU market until July 31, 2024 and be sold until July 31, 2026.
DMDM hydantoin was also the subject of a class action. As a federal court in Illinois summarized the plaintiffs' claims in Castillo v. Unilever, certain TRESemmé shampoos and conditioners contain DMDM hydantoin, and people allergic to it can experience reactions including itchiness, rashes, dermatitis, hair brittleness and hair loss. Those were allegations, and on December 28, 2022 the court granted Unilever's motion to dismiss and dismissed the case with prejudice.
Sources checked September 15, 2026. Every point below links the page it came from.
The sources behind this answer
- Government sourceThe FDA's allergens in cosmetics page lists DMDM hydantoin under formaldehyde and formaldehyde releasing ingredients, alongside bronopol and diazolidinyl urea. US FDA ↗
- Government sourceThe FDA says skin sensitization or allergic dermatitis can develop after repeated contact with formaldehyde-related ingredients, and formaldehyde released into the air can seriously irritate the eyes, nose and lungs. US FDA ↗
- Regulatory filingThe European Commission says certain cosmetic preservatives gradually release formaldehyde to preserve the product, and these formaldehyde releasers are used in both leave-on and rinse-off products. European Commission, Regulation (EU) 2022/1181, via web.archive.org ↗
- Regulatory filingRegulation (EU) 2022/1181 requires the warning releases formaldehyde when total formaldehyde released exceeds 0.001 percent, with earlier-compliant products allowed on the market until July 31, 2024 and on sale until July 31, 2026. European Commission, Regulation (EU) 2022/1181, via web.archive.org ↗
- Legal caseAs summarized by a federal court, plaintiffs in Castillo v. Unilever alleged certain TRESemmé shampoos and conditioners contain DMDM hydantoin and that people allergic to it can experience reactions including rashes, hair brittleness and hair loss. US District Court for the Northern District of Illinois, via GovInfo ↗
- Legal caseOn December 28, 2022 the court granted Unilever's motion to dismiss the second consolidated amended complaint and dismissed the case with prejudice. US District Court for the Northern District of Illinois, via GovInfo ↗
Sources checked September 15, 2026. A brand statement is the company’s own claim, which we report but have not verified. Our grades are model-based estimates, not lab tests.