a boy wondering whether canker sores are contagious

Are Canker Sores Contagious?

Canker sores are small, shallow lesions on the inside of the mouth. Canker sores visibly resemble cold sores, which are caused by the herpes virus and are very contagious, but canker sores are not contagious at all. They cannot be spread from person to person by kissing or sharing food, as cold sores can.

Cold sores also appear on the outside of the mouth and lips, instead of inside the mouth like canker sores. The exact cause of canker sores is unknown, but they seem to appear in response to certain “triggers” such as spicy or acidic foods, small injuries in the mouth, stress, or vitamin deficiencies. They heal on their own and disappear within one to two weeks. Contagious cold sores, on the other hand, will sometimes require antiviral medications before they will be able to heal.

Last Updated: June 16, 2016

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