DMDM hydantoin

DMDM hydantoin is an antimicrobial formaldehyde releaser preservative with the trade name Glydant. DMDM hydantoin is an organic compound belonging to a class of compounds known as hydantoins. It is used in the cosmetics industry and found in products like shampoos, hair conditioners, hair gels, Rite Aid Liquid Lubricant, and skin care products.
DMDM hydantoin slowly releases formaldehyde and works as a preservative by making the environment less favorable to the microorganisms.
Safety.
A patch test study found "An increase in the use of DMDM hydantoin in cosmetic products will also inevitably increase the risk of cosmetic dermatitis in consumers allergic to formaldehyde."