Canker sores are a type of most commonly occurring type of mouth ulcer (aphthous ulcers). Canker sores are different than oral herpes. Canker sores are on the inside of the mouth, whereas the oral herpes are usually on the corners of the mouth. Canker sores are formed on the loose tissues of the mouth at the skin inside the lips and cheeks, the lower part of the mouth, under the tongue and the tonsil area.
The occurrence of canker sores is related to the immune system. There is no medical testing for canker sores. The most common characteristic of canker sore is that it is recurrent. Dentists usually distinguish canker sores from other ulcers by visual inspection.