Most mouth sores are cold sores (also called fever blisters), canker sores, or other irritation caused by:

• Biting your cheek, tongue, or lip
• Chewing tobacco
• Braces
• A sharp or broken tooth or poorly fitting dentures
• Burning your mouth from hot food or drinks

Cold sores are caused by herpes simplex virus and are very contagious. Usually, you have tenderness, tingling, or burning before the actual sore appears. Herpes sores begin as blisters and then crust over.

The herpes virus can reside in your body for years, appearing as a mouth sore only when something provokes it. Such circumstances may include another illness, especially if there is a fever, stress, hormonal changes (such as menstruation), and sun exposure.

Canker sores are NOT contagious and can appear as a single pale or yellow ulcer with a red outer ring, or as a cluster of such lesions. The cause of canker sores is not entirely clear, but may be related to:

• A virus
• A temporary weakness in your immune system (for example, from cold or flu)
• Hormonal changes
• Mechanical irritation
• Stress
• Low levels of vitamin B12 or folate

For unknown reasons, women seem to get canker sores more often than men. This may be related to hormonal changes.

Less commonly, mouth sores can be a sign of an underlying illness, tumor, or reaction to a medication. Such potential illnesses can be grouped into several broad categories:

• Infection (such as hand-foot-mouth syndrome)
• Autoimmune diseases (including lupus)
• Bleeding disorders
• Malignancy (cancer)
• Immunosuppression (that is, when your immune system is weakened -- for example, if you have AIDS or are receiving medication after a transplant).

Drugs that might cause mouth sores include chemotherapeutic agents for cancer, aspirin, barbiturates (used for insomnia), gold (used for rheumatoid arthritis), penicillin, phenytoin (used for seizures), streptomycin, or sulfonamides.

Mouth sores generally last 10 to 14 days, even if you don't do anything. They sometimes last up to 6 weeks. The following steps can make you feel better:

• Gargle with cool water or eat popsicles. This is particularly helpful if you have a mouth burn.
• Avoid hot beverages and foods, spicy and salty foods, and citrus.
• Take pain relievers like acetaminophen.

For canker sores:

• Rinse with salt water.
• Apply a thin paste of baking soda and water.
• Mix 1 part hydrogen peroxide with 1 part water and apply this mixture to the lesions using a cotton swab.

Nonprescription preparations, like Orabase, can protect a sore inside the lip and on the gums. Blistex or Campho-Phenique may provide some relief of canker sores and fever blisters, especially if applied when the sore initially appears.

Additional steps that may help cold sores or fever blisters:

• Apply ice to the lesion.
• Take L-lysine tablets.

Anti-viral medications for herpes lesions of the mouth may be recommended by your doctor. Some experts feel that they shorten the time that the blisters are present, while others claim that these drugs make no difference.

Call your doctor if:

• The sore begins soon after you start a new medication
• You have large white patches on the roof of your mouth or your tongue (this may be thrush or another type of lesion)
• Your mouth sore lasts longer than 2 weeks
• You are immunocompromised (for example, from HIV or cancer)
• You have other symptoms like fever, skin rash, drooling, or difficulty swallowing

Your doctor will perform a physical examination, focusing on your mouth and tongue. Medical history questions may include the following:

• Are the sores on your lips, gums, tongue, lining of your cheeks, or elsewhere?
• Are the sores open ulcers?
• Are there large, white patches on the roof of the mouth or on your tongue?
• How long have you had the mouth sores? More than 2 weeks?
• Have you ever had sores of this type before?
• What medications do you take?
• Do you have other symptoms like fever, sore throat, or breath odor?

Treatment may depend on the underlying cause of the mouth sore.

A topical anesthetic (applied to a localized area of the skin) such as lidocaine or xylocaine may be used to relieve pain (but should be avoided in children).

An antifungal medication may be prescribed for oral thrush (a yeast infection).

An antiviral medication may be prescribed for herpes lesions (although, some feel that this does not shorten the length of time that the lesions are present)

Antibiotics may be prescribed for severe or persistent canker sores.

You can reduce your chance of getting common mouth sores by:

• Reducing stress and practicing relaxation techniques like yoga or meditation
• Avoiding very hot foods or beverages

You can avoid mechanical irritation by:

• Visiting your dentist right away if you have a sharp or broken tooth or misfitting dentures
• Chewing slowly
• Using a soft-bristle toothbrush

If you seem to get canker sores often, talk to your doctor about taking folate and vitamin B12 to prevent outbreaks.

If you get cold sores often, taking L-lysine tablets or increasing lysine in your diet (found in fish, chicken, eggs, and potatoes) may reduce outbreaks. DO NOT use L-lysine if you have high cholesterol, heart disease, or high triglycerides.

To prevent the spread of herpes sores, do not kiss or have oral sex with someone with a cold sore or fever blister. Do not participate in these activities when you have an active cold sore. Do not share razors, lip balm, toothbrushes, or lipsticks.

To prevent cancerous mouth lesions:

• Do not smoke or use tobacco.
• Limit alcohol to 2 drinks per day.
• Wear a wide-brimmed hat to shade your lips. Wear a lip balm with SPF 15 at all times.

Mandell GL, Bennett JE, Dolin R. Principles and Practice of Infectious Diseases. 5th ed. London, UK: Churchill Livingstone, Inc; 2000.

Yeung-Yue KA. Herpes simplex viruses 1 and 2. Dermatol Clin. 2002; 20(2): 249-266.

MacDonald J. Canker sore remedies: baking soda. CMAJ. 2002; 166(7): 884.