Most of us do not understand what the difference is between a canker sore and any other sore in the mouth that is helped by a topical treatment. Can you please explain? Dr. Lee:  Apthous stomatitis is the medical term for canker sores in the mouth (I promise you from here on I will use the term canker sore). Canker sores are very painful oral ulcers.  This is the most common type of benign mouth sore.  The number of sores and size may vary and are spontaneous by nature.  In other words, they can start below the surface and erupt into a sore.  A traumatic ulcer (aren't they all) in the mouth is usually a lesion caused by some sort of outside trauma to the exterior layer of the mouth tissue.  It may be found on the tongue, lips and is usually a solitary benign, but painful lesion.  Frequently, these ulcers can look and feel like a canker sore.  If the lesion persists, have it checked by your dentist. What triggers this type of mouth sore? Dr. Lee:  Psychological stress plays a significant role in recurring canker sores.  Etiologists mention these three known factors as the cause of canker sores - an immune imbalance, a decrease in mucosal barrier (aren't you glad you asked?) or an allergic challenged individual.  In traumatic ulcers there generally is a history of trauma to the mouth.  This can be as simple as biting the inside of your cheek or poking it with a fork. What if my mouth sore persists longer than a week or two? Dr. Lee:  An important part of maintaining good oral hygiene means seeing your dentist on a regular basis.  It also means having your dentist perform an annual comprehensive exam which includes an oral cancer screening.  Any sore in your mouth that you are worried about or if it persists after a round of topical treatment should be examined by your dentist. I've read that bacteria is always present in your mouth. Is there a reason for that? Dr. Lee:  You have good hair days and bad hair days.  Obviously, both good bacteria and bad bacteria also exist.  In optimum health, all of the microbes in your mouth are balanced out.  If not, the result can be: gum disease, decay, ulcerations and even bad breath.  The good news is that with the help of a dentist, the bacteria in the mouth can become balanced for good healthy teeth and bones.