A canker sore appears as a round white spot with a red border. That's what canker sores are like for people who experience them. These shallow ulcers in your mouth can make eating and talking uncomfortable. They may occur on your tongue, on your soft palate, inside your cheeks or lips, and at the base of your gums.

Canker sores differ from cold sores in that they occur in the soft tissues of your mouth and aren't contagious. Conversely, cold sores rarely develop in the soft tissues of your mouth and are extremely contagious because they're caused by the herpes virus.

Canker sores are common, but the cause in many cases is unknown. Canker sores can occur at any age, but often first appear when you are between 10 and 40 years of age.

Often, treatment isn't necessary. Pain often decreases after several days, and canker sores usually heal within one to three weeks. Occasionally, severe canker sores can develop that are larger than 1 centimeter in diameter or last longer than two weeks or both. 

Causes 

The cause of most canker sores remains a mystery.

Researchers generally believe that stress or tissue injury may cause the eruption of canker sores. A minor injury, such as biting the inside of your mouth, may trigger a canker sore. Other causes may include:

• Faulty immune system function

• Nutritional problems such as a deficiency of vitamin B-12, zinc, folic acid or iron

• A disease of the gastrointestinal tract

• Food allergies

• Menstrual periods

Though anyone can get cankers sores, they tend to run in families and may be an inheritable condition.

If you experience any of the following conditions, see your doctor:

• Unusually large sores

• Persistent sores, lasting 3 weeks or more

• Pain that you can't control with self-care measures

• Difficulty drinking enough fluids

• High fever with canker sores

See your dentist if you have sharp tooth surfaces or dental appliances that seem to trigger the sores.