Canker sores are small, painful and persistent ulcers that develop inside the mouth. Although canker sores are small, they can cause simple activities like talking or eating to be extremely painful.

Canker sores, also known as aphthous stomatitis or aphthous ulcers, occur on the tongue, on the soft palate, inside the cheeks or lips, and at the base of the gums. They are round, white or yellow with a red outline, and typically approximately one quarter of an inch in diameter. Canker sores are not synonymous with cold sores because, unlike cold sores, canker sores occur on the internal soft tissues of the mouth and are not contagious.