Canker sores (or mouth ulcers) generally occur inside the mouth. They often are triggered by trauma such as biting your cheek, jabbing your gum with your toothbrush or even overzealous tooth cleaning. Cold sores are tiny, clear, fluid-filled blisters that form around the mouth and are caused by the herpes simplex virus living inside your nerve tissue. (It is usually not the same virus that causes genital herpes.)

Diligence is key when dealing with your oral health, says Academy of General Dentistry spokesperson Eric Z. Shapira, DDS, MAGD. "You need to be aware of any kind of lesion in the mouth or around the mouth, then report it to your dentist so he or she can play 'detective," says Dr. Shapira. "It is important to know what it is and what caused it before it can be treated properly."

Reviewed: January 2012