Surely at one time or another, we have all experienced the irritation of canker sores. Canker sores are small ulcers with a white or gray base and a red border. They appear in the mouth and are not contagious. Factors that may increase the likelihood of a canker sore include fatigue, stress, allergies, biting the cheek or tongue, and reactions to hot food or beverages. Canker sores usually heal on their own after a week or two. If you notice an occurence of one or an more, keep away from spicy, hot, or acidic foods that could irritate the sores.