there are so many causes of mouth ulcers like- -Stress or tissue injury is thought to be the 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. -Sometimes a sharp tooth surface 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 and heat in stomach. . Usually ulcer heals by itself only within 1 week. You can take 'Becosule capsule' once in a day for 1 week for faster recovery. Temporarily you can apply mucopain gel over ulcer for relief..