“Imagine millions of bacteria feasting on particles of food and tissue in our mouth, some giving off smelly waste,” says David Krol, M.D., former chair of the American Academy of Pediatrics Section on Oral Health. As those microbes work to digest milk solids (lactose, proteins, and lipids), they sometimes create excess hydrogen sulfide. This is why, post-cheese, your mouth smells like a mound of rotten eggs.