The upper surface of the tongue is covered by the taste buds. Viewed microscopically, the papillae look like a forest. Individuals with "black hairy tongue" merely have a taller-than-normal forest of papillae. The extra height comes from an accumulation of keratin, which is the chief protein found in hair and skin. Normally, keratin sloughs continuously and gradually from mucous membranes and skin. In people with black hairy tongue, not enough keratin is sloughed off or too much keratin is being produced at the surface of the tongue.
No one knows for certain what causes black hairy tongue, but there are several well-known associations. Black hairy tongue is fairly common among smokers. It is also sometimes seen following treatment with antibiotics. Other associations with black hairy tongue include radiation therapy to the mouth, poor oral hygiene, frequent use of strong mouthwashes and frequent use of antacids.
In most cases, black hairy tongue is more of a cosmetic problem than a functional problem.