Definition Canker sores are small ulcers inside the mouth that are white or grayish with red borders. They are not contagious. Canker sores should not be confused with cold sores, which occur outside the mouth around the lips, are caused by a virus, and are very contagious. Description Canker sores are small patches of raw skin on the tissues that line the cheeks and the inside of the lips. They may also occur at the base of the gums or below the tongue on the floor of the mouth. They may appear as single ulcers or in clusters. Most are about three-eighths of an inch (1 centimeter) wide, but some are larger. Doctors classify canker sores into three groups: minor sores, which are smaller than half an inch (1.3 centimeters) and heal by themselves in seven to ten days; major sores, which are larger than half an inch (1.3 centimeters), are usually deeper than minor sores, may take as long as a month to heal, and may leave scars; and herpetiform sores, which are small, form clusters that ...
