It could be eosinophilic granuloma complex or EGC also known as rodent ulcer. It doesn't have anything to do with rodents actually. It's thought that farmers called it that years ago when their cats developed mouth or lip or chin sores. The farmers believed that diseased rats were biting their cats. The disease is related to allergies or hypersensitivity. It can start when a cat is young. If this is what your kitty has then, your cat probably has an allergy to something it 's touching, inhaling or eating. The best thing to do after taking kitty to the vet, is find out what he's allergic to. If you use plastic food and water dishes, replace them with either stainless steel or ceramic. If the lip improves, then you know the cat is allergic to the plastic or bacteria that can be in the scratches of the plastic bowl. If that doesn't work, change kitty's food. Ask your vet for a good hypoallergenic cat food. Have you moved recently, acquired a new pet, or your cat is under some kind of stress, then changing the environment may cause the lip to clear up if it is stress related. Make sure your kitty doesn't have any fleas. Many cats are allergic to fleas. First things, first, see your vet. Good luck!
