Although the exact cause of most mouth ulcers is unknown, stress or tissue injury is thought to be the main cause of simple mouth ulcers. Certain foods, including citrus or acidic fruits and vegetables (such as lemons, oranges, pineapples, apples, figs, tomatoes and strawberries), can trigger a mouth ulcer or make the problem worse. Sometimes a sharp tooth surface or dental appliance, such as braces or ill-fitting dentures, might also trigger mouth ulcers.
Some cases of complex mouth ulcers are caused by an underlying health condition, such as an impaired immune system; nutritional problems, such as vitamin B-12, zinc, folic acid, or iron deficiency, but they can also be a symptom of Gastrointestinal tract auto immune diseases, such as Coeliac and Crohn’s disease, so its always best to check out what your underlying cause my be.
Some medications, including common pain killers and beta-blockers may cause a reaction that leads to mouth ulcers.