Mouth ulcer. Light micrograph (LM) of a section through a mouth ulcer. Mouth ulcers, also known as canker sores, are normally small, painful lesions that develop in your mouth or at the base of your gums. They can make eating, drinking, and talking uncomfortable. There is no definite cause behind mouth ulcers. However, certain factors and triggers such as stress, injury or infection have been identified. Mouth ulcers also can be a sign of conditions that are more serious and require medical treatment, such as celiac disease, IBS and diabetes. Magnification: x150 when printed at 10 centimetres wide.