Causes of mouth ulcers include: 1. Injurious causes: including physical injury, chemical injury, etc., such as food irritation, chemotherapy, bites, etc.; 2. Infectious causes: including bacterial infection, viral infection, etc.; 3. Digestive system diseases; 4. Reduced immune function: such as fatigue, staying up late, mood swings, etc.; 5. Genetic factors; 6. Endocrine factors; 7. Vitamin and trace element deficiencies.