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The exact cause of most canker sores (aphthous ulcer) is unknown. Stress or tissue injury is thought to be the cause of simple canker sores. Certain foods—including citrus or acidic fruits and vegetables (such as lemons, oranges, pineapples, apples, figs, tomatoes, strawberries)—can trigger a canker sore or make the problem worse. Sometimes a sharp tooth surface or dental appliance, such as braces or ill-fitting dentures, might also trigger canker sores. Some cases of complex canker sores 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 gastrointestinal tract disease, such as Celiac disease and Crohn's disease.