Canker sore: This is most likely a canker sore: a small ulcer, also known as an aphthous ulcer) with a white, yellow or gray center and flat red border. In almost all cases it heals by itself in 7-10 days. The exact cause is not known; genetics may play a role. Fatigue, stress, certain foods affect some people. They are not contagious or precancerous. Numbing ointments or antimicrobial mouthwashes may help.
Answered 9/23/2016
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