Black hairy tongue is caused by overgrowth on the surface of the tongue called hyperkeratosis. The tiny bumps on the surface of your tongue, known as the papillae, contain the same protein that's responsible for the hair on your head. A black and hairy tongue occurs when there's little to no stimulation on the top of your tongue. As a result, the papillae don't shed and can grow to appear hair-like.
These "hairs" can trap food, bacteria, and yeast, creating black, brown, or white discoloration on your tongue. Types of mouthwash, antibiotic treatments, and radiation therapy can also lead to a hairy tongue. Check with your dentist to discuss the best mouthwash and medical treatments for you, considering your health history and the potential side effects.