text: why do we get mouth ulcer?   the exact cause of mouth ulcers is unknown, but in some cases they are thought to be caused by an overreaction by the body's own immune system. factors that appear to provoke them include stress, fatigue, illness, injury from accidental biting, hormonal changes, menstruation, sudden weight loss, food allergies and deficiencies in vitamin b12, iron and folic acid. some drugs, such as nicorandil, have been linked with mouth ulcers.  mouth ulcers are thought to form when the body becomes aware of and attacks chemicals which it does not recognize. the presence of the unrecognized molecules garners a reaction by the lymphocytes, which trigger a reaction that causes the damage of a mouth ulcer.  trauma to the mouth is a common cause of mouth ulcers. physical trauma, such as toothbrush abrasion, poking with sharp food, accidental biting, or dental braces can cause mouth ulcers by breaking the mucous membrane. other factors, such as chemical irritants or thermal injury, may also lead to the development of ulcers. however in many cases, the precise cause is unknown.  a common cause of ulcers is gluten intolerance, in which case consumption of wheat, rye, or barley can result in chronic mouth ulcers. if gluten intolerance is the cause, prevention means taking most breads, pastas, cakes, pies, cookies, scones, biscuits, beers etc. out of the diet and substituting gluten-free varieties where available. artificial sugars, such as those found in diet cola and sugarless gum, have been reported as causes of mouth ulcers as well. they can also be linked to an increased intake of acid such as the acidic form vitamin c or citric acid. in this case the sores disappear after intake decreases or a non acidic form of the vitamin is taken.  other disorders can cause mouth ulcers, including oral thrush, leukoplakia, gingivostomatitis, and oral lichen planus. mouth ulcers are also associated with ulcerative colitis, crohn's disease, coeliac disease (gluten sensitivity), bullous pemphigoid, and behçet's disease. repeat episodes of mouth ulcers can also be indicative of an immunodeficiency, signalling low levels of immunoglobulin in the mucous membrane of the mouth. chemotherapy is a common cause of mouth ulcers. the treatment depends on the believed cause.