Mouth ulcers are caused by a disturbance in the immune system, probably because of bacteria or vira (e.g. the varicella virus). Why this disturbance appears is not known. It is believed to be an autoimmune reaction (a condition in which the body attacks its own tissue).
The ulcers are mostly located on the inner side of the lips and, in any case, they are very uncomfortable. There can be more than one at a time and they can come with different intervals and be of different duration.
In women, the mouth ulcers have a tendency to appear during menstruation, and stress and HIV infections can also provoke them.
A small study has pointed to the fact that toothpaste containing sodium lauryl sulphate increases the risk of developing mouth ulcers.
