A canker sore is a small ulcer or lesion that occurs on the inside of the mouth. They can appear on the top surface or the tip of the tongue, underneath the tongue, on the roof of the mouth or on the gum. Canker sores appear round or oval with a red border and yellow or white center. Canker sores are not contagious.

Canker sores are not the same as cold sores. Cold sores develop on the outside surface of the lips and are a different condition.

A couple of days before you actually see the sore you may feel a tingling or burning sensation in the area where the sore is about to appear.