Canker sores and cold sores (i.e., "fever blisters") are relatively common, sometimes painful conditions with similar symptoms but different causes. Canker sores are painful blisters that form on the inside of the mouth. The cause of canker sores is not known, but the sores may be triggered by injury, stress, smoking, and certain nutrient deficiencies. Canker sores are not contagious (National Institute of Dental and Craniofacial Research, 9/2021).
Cold sores, on the other hand, cause painful blisters on the outside of the mouth. Cold sores are caused by herpes simplex virus (usually type 1 or HSV-1) infection and are very contagious, although not everyone infected with the virus shows symptoms (National Institute of Dental and Craniofacial Research, 9/2021).
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