Canker sores are commonly mistaken for fever blisters or cold sores. The difference is in what causes them and their location. Fever blisters and cold sores are caused by the herpes simplex virus and may occur inside or outside the mouth as well as on the chin or in the nostrils. They often come back in the same location. Fever blisters and cold sores can be spread by kissing.
Canker sores are not contagious and only develop on the inside of the mouth. While both fever blisters and canker sores might look the same with a reddish, inflamed appearance, canker sores might also appear white and usually do not bleed. Younger women tend to get canker sores more often than older women.