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An aphthous ulcer ( /ˈæfθəs/ AF-thəs), also known as a canker sore, is a type of mouth ulcer that presents as a painful open sore inside the mouth or upper throat characterized by a break in the mucous membrane. Its cause is unknown, but they are not contagious. The condition is also known as aphthous stomatitis (stomatitis is inflammation of the mucous lining), and alternatively as Sutton's Disease, especially in the case of major, multiple, or recurring ulcers.

The term aphtha means ulcer; it has been used for many years to describe areas of ulceration on mucous membranes. Therefore, the term aphthous ulcer is redundant and the term aphthous stomatitis is preferred. Aphthous stomatitis is a condition characterized by recurrent discrete areas of ulceration that are almost always painful. Recurrent aphthous stomatitis (RAS) can be distinguished from other diseases with similar-appearing oral lesions, such as certain oral bacteria or herpes simplex, by their tendency to recur, and their multiplicity and chronicity. Recurrent aphthous stomatitis is one of the most common oral conditions. At least 10% of the population has it, and women are more often affected than men. About 30–40%

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