Q. I've seen it in a couple of lotions that I'm using. Is there anything "bad" about hydantoin? -Bethany
A. The EWG Skin Deep Cosmetics Database rates it a "0" risk igredient, however, hydantoin has been found in numerous studies to cause birth defects.
This study found that hydantoin interferes with Folic acid, a key nutrient for the proper fetal development in humans (and most mammals.) (Source) It has been found to create abnormalities in the urinary tract in human fetuses. In mice studies it disrupted the development of the muscosceletal system, ear development, and other abnormalities. (Source) (Source)
The doses that affect development are relatively high (like taking a pill of it every day) and there are no studies on absorption of this chemical through skin, and if dermal application poses a risk. Hydantoin can be a skin irritant in higher concentrations. All-in-all, I would avoid this ingredient, especially if pregnant.