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What Are Canker Sores?

Canker sores, also known as aphthous ulcers, are annoying irritations inside the mouth that afflict as many as 20% of Americans. They are different from cold sores, which usually appear on the outside of the mouth on the lips. They are also different from traumatic ulcers, which look like canker sores but are caused by trauma such as rough dentures, a slip of the toothbrush, or hot food.

Canker sores appear most commonly in adolescents and young adults, often occurring during times of stress. They are also more likely to appear around a women’s menstrual period, and there is a greater likelihood of having canker sores if a parent also has them.

What Causes Canker Sores?

No one knows what causes canker sores, or why women are more likely to get them. Outbreaks often seem related to physical or emotional stress. Certain foods such as acidic foods, tomatoes, eggplant, and possibly gluten, may also cause them to develop. Ingredients in toothpaste can trigger a canker sore.

Unlike cold sores, canker sores are not contagious. They don’t pose any health risk other than the discomfort that comes along with them.

How Can I Prevent Canker Sores?

Canker sores cannot be prevented. Many techniques have been proposed, but there is little evidence they work.

Medical Reference Reviewed by Sabrina Felson, MD on May 10, 2019


The Mayo Clinic.

National Institutes of Health.

Goldman’s Cecil Medicine, 24th ed.

Chattopadhyay A. Otolaryngol Clin North Am., Feb. 2011.