Canker sores are open lesions (ulcers) that appear inside the mouth. They are round, shallow and small (usually less than 1 centimeter in diameter) and tend to occur on the inner cheeks, inside the lip, at the base of the gums or underneath the tongue. They are usually preceded by a tingling or burning sensation, and typically appear as a pale-colored crater with a red border.

Canker sores are different from, but may be confused with, cold sores – contagious blisters that appear outside the mouth (usually around the lip area) that burst and crust over.

The cause of canker sores is unknown, but is believed to be related to dysfunction of a person’s immune system. Certain factors appear to make a person more susceptible to canker sores (e.g., gender, age, family history of canker sores). Conditions that may trigger an outbreak of canker sores include injury to the tissue lining of the mouth, stress, allergies and vitamin deficiency. Certain diseases (e.g., cancer, digestive disorders, HIV/AIDS) may also be associated with canker sores.

Patients who experience canker sores that are very large, extremely painful, recur frequently or have not healed after two weeks should seek medical attention. 

Canker sores can usually be diagnosed by visual inspection of the affected area by a dentist or other health care professional. A medical history may be taken to identify potential causes of the sores. Occasionally, additional tests (e.g., blood tests, biopsy) may be performed in order to rule out certain conditions or diseases.

In most cases, canker sores heal without treatment within approximately two weeks. There is no known cure for canker sores. Treatment remedies focus on alleviating a patient’s symptoms. These remedies can include avoiding foods or beverages that irritate the ulcers, as well as taking medications (e.g., mouth rinses, topical anesthetics and over-the-counter pain medications) to reduce the pain and discomfort of canker sores. Home remedies are also available.