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Canker sores appear inside the mouth. They usually are small ulcers (minor aphthous ulcers) with a white, yellow or gray center and a flat red border. Rarely, canker sores can be very large (major aphthous ulcers) with a raised border. There may be one or several ulcers and they recur at varying periods of time.
A canker sore usually begins as a red spot or bump. It may produce a tingling or burning sensation before other symptoms appear. Canker sores are painful. Fortunately, most canker sores heal spontaneously in 7 to 10 days.
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