Dark discoloration of the tongue accompanied by an overgrowth of papillae (tiny, circular bumps) on the upper surface of the tongue. The condition is not considered medically serious and may be caused by any one of several factors. Antibiotic treatment, especially tetracycline, can cause the bacteria normally present in the mouth to multiply and accumulate in the creases of the tongue. This gives the tongue a darkened appearance and causes the papillae to become extended and appear hair-like. Black hairy tongue may also be related to a fungal infection of the mouth or to using over-the-counter antacid medications containing bismuth. Some people’s tongues have very deep crevices and cracks that trap food, drink, and other substances. For these people, drinking tea or coffee, chewing tobacco, or smoking may cause black or dark brown discoloration of the tongue.