Any break in the lining of the mouth is considered an ulcer. The inside of the mouth is incredibly sensitive and a break will expose the nerves that lie in and underneath it - the mouth ulcer is an exposed nerve.
The common mouth ulcer (minor aphthous) affects about 25% of the population, on occasion, and usually appears as a white, yellow or grey oval with an inflamed red edge. They usually appear inside the lips or cheeks, around or below the tongue - either singly or in crops of three or four. Although they should heal within 7-10 days, and are neither dangerous nor infectious, ulcers can be extremely painful, especially when agitated.
Injury to the mouth is a common cause of ulcers; accidental biting, or a sharp edge on a tooth damage the tissue of the mouth resulting in an ulcer. They may also be linked with general health issues such as hormonal imbalances, puberty, anaemia or diabetes and are often associated with stress and infection from viruses. They can be made worse by inflammation or a secondary infection.
Far less common is the major aphthous ulcer. Larger and usually occurring singly, they can last for up to a month, often leaving a small white scar.
Babies or young children may have ulcers as a result of an acute herpes infection and this condition usually clears up within 10 days.
There are other more serious causes of ulcers, including inflammatory bowel disease and immune disorders, but these are usually accompanied by other symptoms around the body.
The most serious mouth ulcer is that caused by oral cancer. Smoking, especially if combined with the heavy use of alcohol, significantly increases the likelihood of developing mouth cancer. Any mouth ulcer that does not heal within two weeks should be assessed by a dentist or a doctor without delay.