Canker sores are not caused by herpes. Fever blisters, another common mouth ailment similar to canker sores, are caused by herpes. Fever blisters are also known as cold sores and usually occur outside the mouth–on the lips, chin, cheeks or in the nostrils. When fever blisters do occur inside the mouth, it is usually on the gums or the roof of the mouth. Fever blisters are smaller than canker sores, heal more quickly, and often begin as a blister. Canker sores commonly occur only in the mouth, and are often the result of poor dental hygiene and other mouth irritations, but not by herpes.

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