DefinitionA canker sore is a painful, open sore in the mouth. Canker sores are white or yellow and surrounded by a bright red area. They are benign (not cancer).See also: Herpes, Fever blisters and Canker soresAlternative NamesAphthous ulcer; Ulcer - aphthousCauses, incidence, and risk factorsCanker sores are a common form of mouth ulcer. They occur in women more often than men. They may occur at any age, but usually first appear between the ages of 10 and 40.Canker sores usually appear on the inner surface of the cheeks and lips, tongue, soft palate, and the base of the gums.Canker sores can run in families. They may also be linked to problems with the body's immune (defense) system. The sores may occur after a mouth injury due to dental work, aggressive tooth cleaning, or biting the tong