Published on May 10, 2024 at 8:00 / Updated on May 25, 2024 at 8:00

(mouth ulcers)

A canker sore is a shallow, crater-shaped sore that develops on the tongue or on the inside of the lip or cheek. It usually has a red border and a white or yellowish centre. A canker sore can be small or large in size (> 1 cm), and there may be more than one sore at a time.

While canker sores are not contagious, they tend to run in families. Anyone can get a canker sore, but they occur more commonly in women, adolescents, and young adults.

Causes and triggers

The exact cause of canker sores is unknown, but it is thought that they may be related to stress or to injuries to the inside of the mouth (e.g., accidentally biting yourself). The following factors may also contribute to canker sores:

  • Certain systemic diseases (e.g., inflammatory bowel diseases)
  • Hormonal changes
  • Use of certain medications
  • Vitamin deficiency (e.g., vitamin B12)
  • Highly acidic foods (citrus, tomatoes, etc.)


Canker sores can be painful and can make it difficult to speak or eat, but they usually heal on their own within 10 to 14 days. Here are some tips to help relieve the pain associated with canker sores and to promote healing:

  • Avoid foods that are hard, pointy, salty, acidic, crunchy, or hard to chew.
  • Carefully brush teeth and gums without irritating the sore (the use of a soft-bristled toothbrush is recommended).
  • After each meal, rinse mouth out with mouthwash (without swallowing), and avoid mouthwash that contains alcohol. Here are two easy recipes for homemade mouthwash:
    • Salt water mouthwash: dilute 1/2 teaspoon to 1 teaspoon (2.5-5 mL) salt in one cup (250 mL) warm water.
    • Hydrogen peroxide mouthwash: mix hydrogen peroxide 3% with an equal part of water (e.g., 50 mL hydrogen peroxide 3% with 50 mL water).

Your pharmacist may recommend a product that can be applied directly to the canker sore, which provides protection when eating (e.g., Orabase). A numbing gel (e.g., Orajel) or an oral pain reliever (e.g., Tylenol) may also be recommended to help relieve pain. To prevent new outbreaks, the following measures are recommended:

  • If the canker sores are caused by accidentally biting the inside of your mouth, make sure you chew food slowly and carefully.
  • Make sure that braces and dentures are properly adjusted to prevent irritation.
  • Reduce stress.

When should I see a medical professional?

  • If there is no improvement after 14 days.
  • If the sore is growing, or if other sores develop.
  • If you are unable to control the pain using the recommended medications.
  • If you feel ill, have a fever, or have difficulty swallowing.
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