A canker sore (aphthous ulcer) is a common form of mouth ulcer. It is a benign open sore in the mouth, which appears as a painful white or yellow sore (ulcer) surrounded by a bright red area. The cause is unknown.
Canker sores occur in women more often than men. They may occur at any age, but usually first appear between the ages of 10 and 40.
There are multiple types of canker sores: