DMDM Hydantoin
In plain terms
A preservative that protects products from bacteria and mold by slowly releasing small amounts of formaldehyde over time.
What the evidence shows
DMDM hydantoin is a preservative that works by slowly releasing formaldehyde into the product. The released formaldehyde is the issue, and in classification terms it is not a small one: the IARC and the US National Toxicology Program both list formaldehyde as a known human carcinogen, and it is a recognized skin sensitizer. The practical picture is more measured, since the amount a preservative releases is small and formaldehyde's clearest cancer risk comes from breathing it at higher exposures, not from a rinse-off product. What is well established is the contact-allergy side: formaldehyde-releasing preservatives are among the more common preservative allergens, and several US states have moved to bar this class from cosmetics. Formaldehyde is never named on the label here, because it is released rather than added, so the preservative name itself is the signal.
On the label
It appears among the preservatives in the second half of the ingredient list. The label will say DMDM hydantoin, never formaldehyde.