A mouth ulcer (also termed an oral ulcer, or a mucosalulcer) is an ulcer that occurs on the mucous membrane of the oral cavity. More plainly, a mouth ulcer is a sore or open lesion in the mouth.

Mouth ulcers are very common, occurring in association with many diseases and by many different mechanisms, but usually there is no serious underlying cause.

Most are self-resolving and transient (e.g. after a simple cheek bite). However, some may require the intervention of a medical or dental practitioner. The diagnosis of the more common oral ulcerations, in another wise healthy patient, is straightforward and determined from the medical history and clinical examination.

Ulcers caused by local trauma are painful to touch and sore. They usually have an irregular border with erythematous margins and the base is yellow.

However, patients with impaired immunological function (e.g. HIV, chemotherapy, malnutrition) may present with more severe, widespread, atypical presentations that require a comprehensive assessment.

The diagnosis comes mostly from the history and examination, but the following special investigations may be involved: blood tests (vitamin deficiency, anemia, leukemia, Epstein-Barr virus, HIV infection, diabetes) microbiological swabs (infection,) or urinalysis (diabetes.)

A biopsy (minor procedure to cut out a small sample of the ulcer to look at under a microscope) with or without immunofluorescence may be required, to rule out cancer, but also if a systemic disease is suspected.

Treatment is cause related, but also symptomatic if the underlying cause is unknown or not correctable. It is also important to note that most ulcers will heal completely without any intervention. Treatment can range from simply smoothing or removing a local cause of trauma, to addressing underlying factors such as dry mouth or substituting a problem medication.