Canker Sore

Front view of face with fingers pulling down lower lip to show aphthous ulcers.

A canker sore is a painful sore on the lining of the mouth. It’s also called an aphthous ulcer. It is most painful during the first few days, and it lasts about 7 to 14 days before going away.


Canker sores are not cold sores or fever blisters. They are not contagious, so they are not spread by contact. The exact cause of canker sores is not clear, but there are various things that can trigger them in different people.

  • Mild injury, such as biting the inside of the mouth, lip, or cheek, or dental procedures

  • Stress

  • Poor diet, or lack of certain nutrients, including B vitamins and iron

  • Foods that can irritate the mouth, including tomatoes, citrus fruits, and some nuts (foods that are acidic or contain bitter substances called tannins)

  • Irritating chemicals, such as those in some toothpastes and mouthwashes

  • Some chronic illnesses


Canker sores occur on the lining of the mouth. They can be inside the cheeks or lips, on the roof of the mouth, at the base of the gums, on the tongue, or in the back of the throat. Canker sores:

  • Are small and flat (not raised)

  • Can be white or yellowish bumps that are red around the edges or have a red halo

  • Are usually small in size, roundish, and in groups

  • Cause pain or burning

Canker sores don't leave a scar. But they usually come back.

Home care

The goals of canker sore treatment are to decrease the pain, speed healing, and prevent sores from coming back. No single treatment works for everyone. Try different methods to see what works best.

General care

  • You may find that soft, easy-to-chew foods cause less pain.

  • Drink with a straw to direct liquids away from the sore.

  • Use a soft-bristle toothbrush, and brush your teeth gently.

  • Don't eat acidic, salty, or spicy foods. These may irritate the canker sores.

  • Don't eat rough or crunchy foods such as crusty bread or potato chips. These kinds of foods can hurt the inside of your mouth or scrape your canker sores.


You can try over-the-counter medicines that cover the sores and numb them. This protects the sores while they heal and helps reduce pain.

Homemade rinses and solutions

You can use these solutions as mouth rinses. Spit them out after using them. You can also dab them on the sores. You can repeat these treatments as often as needed.

  • Rinse your mouth with 1/2 teaspoon of salt in 1 glass of warm water.

  • Mix equal amounts of hydrogen peroxide and water. You can use this as a mouthwash or dab it on sores with a cotton swab. You can also add sodium bicarbonate to this to make a paste, and then dab it on sores.

Follow-up care

Follow up with your healthcare provider, or as advised. If a culture was done, you can call as directed for the results. You will be told if your treatment needs to be changed.

Call 911

Call 911 if you have any of these:

  • Trouble breathing

  • Inability to swallow

  • Extreme drowsiness or trouble waking up

  • Fainting or loss of consciousness

  • Fast heart rate

  • Seizure

  • Stiff neck

When to seek medical advice

Call your healthcare provider right away if any of these occur:

  • You have a fever of 100.4°F (38°C) or higher, or as directed by your provider

  • You are pregnant

  • You just had surgery or another medical procedure, or you were just discharged from the hospital

  • It's too painful to eat or swallow

  • The sore does not go away within 2 weeks

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