Canker sores, mouth ulcers with a white or gray base and a red border, seem to appear out of nowhere. Women get them more frequently than men, and though the cause is unknown, some experts believe the culprit may be a problem with the body’s immune system. Fatigue, stress or allergies increase your risk, says the National Women’s Health Information Center, as does a cut inside your cheek or on your tongue. Treat a sore with a mild mouthwash or salt water or swab the blemish with milk of magnesia or a mixture of hydrogen peroxide and water. You can’t prevent canker sores, but if you get them often, talk with your dentist or dental hygienist.