Mouth ulcers, also known as canker sores, are small and painful sores that appear in the cheeks, lips, and gums. Common causes include aggressive brushing, minor injury, recent dental work, problems with the immune system, celiac disease, inflammatory bowel disease, sensitivity to acidic or spicy foods, mouthwashes and toothpaste containing lauryl sulfate, as well as diets low in zinc, iron, folic acid, or vitamin B12. In most cases, you do not need any treatment for mouth ulcers as they will disappear on their own. If the symptoms persist, your doctor would suggest a prescription mouthwash with lidocaine or dexamethasone. Some OTC liquids, pastes, or gels can also help.