DefinitionA canker sore is a painful, open sore in the mouth. Canker sores are white or yellow and surrounded by a bright red area. They are not cancerous.A canker sore is not the same as a fever blister (cold sore).Alternative NamesAphthous ulcer; Ulcer - aphthousCauses, incidence, and risk factorsCanker sores are a common form of mouth ulcer. They may occur with viral infections. In some cases, the cause cannot be determined.Canker sores may also be linked to problems with the body's immune (defense) system. The sores may occur after a mouth injury due to dental work, aggressive tooth cleaning, or biting the tongue or cheek.Canker sores can also be triggered by:Emotional stressLack of certain vitamins and minerals in the diet (especially iron, folic acid, or vitamin B-12)Menstrual periodsHormonal changesFood allergiesAnyone can develop a canker sore. Women are more likely to get them than men. Canker sores may run in families.SymptomsCanker sores usually appear on the inner surface of the cheeks and lips, tongue, soft palate, and the base of the gums.Symptoms include:One or more painful, red spots or bump that develops into an open ulcerMiddle of the sore is white or yellowUsually small (under 1 cm) but occasionally largerSore may turn gray just before starting to healLess common symptoms include:FeverGeneral discomfort or uneasiness (malaise)Swollen lymph nodesPain usually goes away in 7 to 10 days. It can take 1 to 3 weeks for a canker sore to completely heal. Large ulcers can take longer to heal.Sometimes, a severe outbreak of canker sores may be accompanied by nonspecific symptoms of illness, such as fever.Signs and testsYour health care provider can often make the diagnosis by looking at the sore.If canker sores persist or continue to return, tests should be done to look for other causes, such as erythema multiforme, drug allergies, herpes infection, and bullous lichen planus.A biopsy may be used to distinguish a canker sore from other causes of mouth ulcers.Canker sores are not cancer and do not cause cancer. There are types of cancer, however, that may first appear as a mouth ulcer that does not heal. See: Squamous cell carcinoma.TreatmentTreatment is usually not necessary. In most cases, the canker sores go away by themselves.If you have a canker sore, you should not eat hot or spicy foods, which can cause pain. Mild, over-the-counter mouth washes or salt water may help. There are over-the-counter medicines that soothe the painful area. These medicines are applied directly to the sore area of the mouth.The easiest home remedy is a mixture of half hydrogen peroxide and half water. Use a cotton swab to apply the mixture directly to the canker sore. Then, dab a small amount of Milk of Magnesia on the canker sore, three to four times a day. This is soothing and may also help it heal.Another home remedy is to mix half Milk of Magnesia and half Benadryl liquid allergy medicine. Swish this mixture in your mouth for about 1 minute, then spit it out.Prescriptions may be required for severe cases. This may include fluocinonide gel (Lidex) or chlorhexidine gluconate mouthwash. Powerful anti-inflammatory medicines called corticosteroids are sometimes used.To prevent bacterial infection, brush and floss your teeth regularly and get routine dental check-ups.Expectations (prognosis)Canker sores usually heal on their own. The pain usually decreases in a few days. Other symptoms disappear in 10 to 14 days.ComplicationsAntibiotic treatment for canker sores may lead to oral thrush (a type of mouth infection) or other Candida infections. Rarely, bacterial infections such as cellulitis and Ludwig's angina may occur.Canker sores are not cancer and don't lead to cancer. But if you have a mouth ulcer lasts more that 2 weeks, you should see your doctor to rule out possible cancer.Calling your health care providerApply home treatment and call your health care provider if symptoms of canker sores persist or worsen, or canker sores recur more often than 2 or 3 times per year.Call your health care provider if symptoms are associated with other problems such as fever, diarrhea, headache, or skin rash.ReferencesMunoz-Corcuera M, Esparza-Gomez G, Gonzalez-Moles MA, Bascones-Martinez A. Oral ulcers: clinical aspects. A tool for dermatologists. Part I. Acute ulcers. Clin Exp Dermatol. 2009 Apr;34(3):289-94.Munoz-Corcuera M, Esparza-Gomez G, Gonzalez-Moles MA, Bascones-Martinez A. Oral ulcers: clinical aspects. A tool for dermatologists. Part II. Chronic ulcers. Clin Exp Dermatol. 2009 Jun;34(4):456-61. Epub 2009 Apr 14.Reviewed ByReview Date: 02/17/2011Linda J. Vorvick, MD, Medical Director, MEDEX Northwest Division of Physician Assistant Studies, University of Washington, School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.