An abnormal patch of white or gray tissue that usually forms on the inside of the mouth, especially on the gums, tongue, inside of the cheek, and under the tongue. The patch does not come off when scraped. Leukoplakia is most commonly caused by smoking or chewing tobacco, but may also be caused by heavy alcohol use, poorly fitting dentures, or jagged, broken, or sharp teeth that rub against the tissues in the mouth. Most leukoplakias are benign (not cancer), but some may be precancerous or an early sign of cancer.