A canker sore begins as a red, ulcerated spot with a yellowish border. The ulcer then becomes covered by a coagulated yellowish mixture of fluids, bacteria, and white blood cells. The development of the sore may be preceded by a burning and tingling sensation.
Canker sores range in size from as small as a pinhead to as large as a quarter.
They appear suddenly and often leave suddenly, usually lasting from four to twenty days. Some experts believe that these painful mouth ulcerations are contagious, but others disagree. Canker sores occur most often in females. They may result from an abnormal immune response to normal bacteria in the mouth. Canker sores are occasionally associated with Crohn's disease, which affects the bowels.