What is a canker sore?

Canker sores, also called aphthous ulcers or mouth ulcers, are small, round, or oval sores that develop inside the mouth. These painful ulcers have a yellow, white, or gray center and are surrounded by a red border. Canker sores can occur on the inside of the lips, cheeks, palate, tongue, or gums. Although canker sores are not usually serious and generally go away on their own within one or two weeks, they can be painful and can make eating, drinking, and talking difficult.