A mouth ulcer is an ulcer that occurs on the mucous membrane of the oral cavity.Mouth ulcers are very common, occurring in association with many diseases and by many different mechanisms, but usually there is no serious underlying cause.Encyclopedia Entry for Mouth Ulcer :Mouth ulcers. Mouth ulcers are caused by many disorders. These include: Canker sores Gingivostomatitis Herpes simplex ( fever blister ) Leukoplakia Oral cancer Oral lichen planus Oral thrush A skin sore caused by histoplasmosis may also appear as a mouth ulcer.Symptoms will vary, based on the cause of the mouth ulcer. Symptoms may include: Open sores in the mouth Pain or discomfort in the mouth.Most of the time, a health care provider or dentist will look the ulcer and where it is in the mouth to make the diagnosis. You may need blood tests or a biopsy of the ulcer may be needed to confirm the cause.The goal of treatment is to relieve symptoms. The underlying cause of the ulcer should be treated if it is known. Gently cleaning your mouth and teeth may help relieve your symptoms. Medicines that you rub directly on the ulcer. These include antihistamines, antacids, and corticosteroids that may help soothe discomfort. Avoid hot or spicy foods until the ulcer is healed.The outcome varies depending on the cause of the ulcer. Many mouth ulcers are harmless and heal without treatment. Some types of cancer may first appear as a mouth ulcer that does not heal.Complications may include: Cellulitis of the mouth, from secondary bacterial infection of ulcers Dental infections ( tooth abscesses ) Oral cancer Spread of contagious disorders to other people.Call your provider if: A mouth ulcer does not go away after 3 weeks. You have mouth ulcers return often, or if new symptoms develop.To help prevent mouth ulcers and complications from them: Brush your teeth at least twice a day and floss once a day. Get regular dental cleanings and checkups.Oral ulcer; Stomatitis - ulcerative; Ulcer - mouth.Oral thrush Oral thrush Canker sore (aphthous ulcer) Canker sore (aphthous ulcer) Lichen planus on the oral mucosa Lichen planus on the oral mucosa Mouth sores Mouth sores.Daniels TE, Jordan RC. Diseases of the mouth and salivary glands.