DMDM hydantoin is a preservative in cosmetics and personal care products. It works by slowing and preventing spoilage in products such as shampoos and hair conditioners, and in skin care products like moisturizers and makeup foundations. DMDM hydantoin is also an antimicrobial agent used in cosmetics and personal care products. As an antimicrobial, it can help prevent the growth of fungi, yeast and harmful bacteria that can make people sick or give them rashes, for example. DMDM hydantoin is a “formaldehyde donor,” which means that to work as a preservative and antimicrobial, it releases small levels of formaldehyde throughout the shelf-life of a personal care product or cosmetic product.