Canker Sores (Aphthous Ulcers)

Canker sores are ulcers that affect the mucous membranes inside the mouth (inner cheeks, gums or lips and, occasionally, the tongue). They may occur singularly or in groups. There are two forms: minor, with a flat border, small, lasting 7-10 days, and major, with a raised border, large, (lasting up to 2 weeks). The major form occurs rarely.

In contrast, “cold sores” or “fever blisters” usually erupt on the lips or the skin around the lips. They sometimes occur on the gum tissue near the teeth and/or on the palate (roof of the mouth). Recurrences may be triggered by wind, sun, fever, stress, or other irritants. Cold sores are caused by the herpes simplex virus and are contagious.

Before it becomes visible, the canker sore may produce a tingling or burning sensation. Canker sores usually begin as a red spot or bump. After 6-24 hours, the ulcer appears as a small, round depression 1/8" to 1/4" in diameter, surrounded by a reddened area of inflammation. The center is often grayish-white, due to a layer of dead cells - but underneath this layer, the tissue is extremely red and raw looking.

What causes canker sores?

The exact cause is not known. Fatigue, emotional stress, and certain foods may increase the possibility in some people. It may be physical damage such as a scratch by a nut or a stray toothbrush bristle, a bruise or a burn from hot food. Some people avoid chocolate, walnuts or acidic foods such as citrus fruit, tomatoes and vinegar because they seem to trigger the ulcers. White blood cells (lymphocytes) may affect the lining of the mouth, triggering these painful sores. Canker sores are not contagious or pre-cancerous. A viral agent has not been established for either of the forms.

Who is susceptible?

Women are twice as likely to have canker sores as men. People whose parents have canker sores have a 90% chance of developing them early in life.


There is nothing that will stop canker sores from recurring. Two cleansing antiseptic mixtures are: 1) hydrogen peroxide diluted with water to half-strength and 2) one teaspoon each of salt and baking soda to four ounces of water. These should be used as a rinse or gargle four times a day. Over-the-counter topical medications (such as numbing agents and protective pastes/discs.) and anti-microbial mouthwashes may relieve the discomfort and help to prevent an infection in the area. It is also helpful to avoid hot, spicy, or acidic foods and beverages that may irritate the sores.

If the sores have lasted more than two weeks or, in especially severe cases when oral pain precludes eating and drinking, a visit to a clinician is indicated. A prescription topical paste containing a steroid (similar to prednisone) can offer further relief by controlling the inflammatory effect of lymphocytes.

Mouth sores can be caused by other more serious things. If you have a sore that does not fit the description above, it should be evaluated by your medical provider, dentist, or oral surgeon - particularly if you engage in oral sex or chew tobacco.


American Dental Association web site,, search for canker sore
Cecil's Textbook of Medicine, 23rd Ed. (2008). Diseases of the Mouth and Salivary Glands.