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Canker Sores (143)

Mouth Ulcers • Aphthous Ulcers

The Facts (6)

Canker sores are small sores that appear on the inside of the mouth, especially the cheeks, the inside of the lips, and the tongue. They're also called aphthous ulcers. Rarely, they may develop on the gums or on the roof of the mouth. Small canker sores disappear within 10 days and don't scar. Large ones are less common, take weeks to heal, and can leave scars. Canker sores aren't contagious like cold sores are.

At any given time, 20% of Canadians will have a canker sore. They're most common in women and in people aged 10 to 40 years. Many people get them regularly, at least once a year. In the most severe cases, people get one after another.


Doctors don't know exactly what causes canker sores. They may be hereditary, but doctors have not figured out exactly how people inherit the tendency to get them. Researchers think they may be an overreaction to the Streptococcus bacteria, because the bacteria are often found in the canker sore. People with canker sores often have small injuries from dental injections and toothbrushes in the lining of the mouth.

Canker sores often occur near the time of a menstrual period. They may also be brought on by stress - for example, many students get them during exams. Other possible causes include lack of cyanocobalamin (vitamin B12), folic acid, or iron. Certain foods may be triggers for canker sores or act as nutrients for the Streptococcus bacteria, including coffee, chocolate, cheese and tomatoes. Rarely, they are associated with celiac disease, Crohn's disease, Behcet's syndrome, or HIV.

Symptoms and Complications

A canker sore appears as a round white spot with a red border. It forms on the soft tissues in the mouth, such as inside the lip or cheek, or on the roof of the mouth. Small ones are about 2  mm to 3 mm across and often come in groups. Large ones have an irregular shape.

Some people may feel burning or tingling up to 48 hours before a canker sore appears. Canker sores cause a lot of pain that lasts between 4 and 10 days. The pain is worsened by hot or spicy foods that touch the sore.

Complications of canker sores can include fever, difficulty swallowing, scarring, swollen glands, and feeling run-down. Sores that keep coming back can indicate that important vitamins may be low or that there is an underlying health problem.

Fortunately, canker sores do disappear over time and no further health risks are associated with them.

Making the Diagnosis

Herpes simplex sores (cold sores) look like canker sores, but usually a dentist or doctor can diagnose canker sores by their shape, size, and location. Canker sores are always found inside the mouth, whereas cold sores are usually found on the lips. Canker sores cause a lot of pain for a sore that is quite small. A doctor may test for other health problems (e.g., low levels of folic acid or vitamin B12 in the blood) if sores keep returning.

Treatment and Prevention (7)

Canker sores usually heal by themselves within 14 days without any treatment. Various treatments are only useful to relieve the pain of the sores. People with canker sores can rinse their mouth with salt water. Avoiding hot and spicy foods also helps to minimize pain.

You can help prevent canker sores by making sure you're not deficient in folic acid or vitamin B12. If injury or irritation to the mouth causes canker sores, it is important to remove any sources of irritation, such as ill-fitting dentures.

There are many pain relievers for canker sores. Viscous lidocaine* is an anesthetic that can be applied to the sore or used to rinse the mouth to numb the pain. Although it relieves pain, it may interfere with a person's sense of taste. Carboxymethylcellulose is a protective coating that can also be put on the sore to relieve pain. Benzydamine mouthwash can provide temporary relief from the pain of canker sores. None of these options speed up healing. Silver nitrate can also be applied to the sore to relieve pain, but it may cause discoloration where it is applied and may delay healing.

If you tend to have a lot of canker sores, or they keep coming back, it is a good idea to see a dentist or doctor. The dentist or doctor can prescribe an antibacterial mouthwash that can be used as soon as a new sore starts developing.

Severe canker sores may be rubbed with a corticosteroid ointment, available by prescription only. They may also be treated with a medication called dexamethasone in a mouth rinse, or prednisone taken as tablets.

If you have continuous or very severe canker sores, it is best to see a specialist in oral medicine. Treatment may require long-term use of steroids, immunosuppressants, or injections of steroids into the lesions.

*All medications have both common (generic) and brand names. The brand name is what a specific manufacturer calls the product (e.g., Tylenol®). The common name is the medical name for the medication (e.g., acetaminophen). A medication may have many brand names, but only one common name. This article lists medications by their common names. For information on a given medication, check our Drug Information database. For more information on brand names, speak with your doctor or pharmacist.


  1. Berkow, Robert, M.D., Beer, Mark H. M.D., Fletcher, Andrew J. (Eds.). The Merck Manual of Medical Information: Home Edition. White Station, N.J: Merck Research Laboratories. 1997; 498.
  2. Disorders of the Oral Region. In; The Merck Manual 17th ed. Merck Research Laboratories 1999:751-759
  3. Komaroff, Anthony L., M.D. (Ed.). Harvard Medical School Family Health Guide. New York: Simon and Schuster, Inc., 1999; 487.
  4. Morgan, Peter, M.D. The Canadian Medical Association Home Medical Encyclopedia. Montreal: The Reader's Digest Association (Canada) Ltd., 1992; 1021.
  5. Nonprescription Drug Reference for Health Professionals: Canadian Pharmaceutical Association 1996
  6. University of Toronto, Faculty of Dentistry. Aphthous Ulcers. http://www.utoronto.ca/dentistry/newsresources/evidence_based/aphthousulcers.pdf. Accessed Feb. 28, 2009.
  7. Recurrent Aphthous Stomatitis. The Merck Manual for Healthcare Professionals [Online]. Available from Merck Manuals Online Medical Library.  Last updated August 2014.  Accessed January 11, 2016.  Available at http://www.merckmanuals.com/professional/dental-disorders/symptoms-of-dental-and-oral-disorders/recurrent-aphthous-stomatitis
  8. Hennessy BJ. Recurrent aphthous stomatitis. Merck Manual Professional Version. Last modified Sep 2018. Available at: https://www.merckmanuals.com/en-ca/professional/dental-disorders/symptoms-of-dental-and-oral-disorders/recurrent-aphthous-stomatitis. Accessed Apr 7, 2020.

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