There are multiple causes of black hairy tongue (lingua villosa nigra) …

poor oral hygiene / dry mouth / mouth breathing / general debilitation / oxygenating or oxidizing mouthwash / soft, non-abrasive food / fasting / tobacco chewing or smoking / betel chewing / drug abuse (alcohol, crack cocaine, heroin) / fever / chemotherapy / head or neck radiation / pellagra / AIDS / etc.

Crispian Scully et al (Oral Medicine and Pathology at a Glance, 2010) listed some “Drugs that may cause black hairy tongue” …

“antibiotics (broad spectrum and griseofulvin) / bismuth-containing preparations / chlorhexadine / corticosteroids / hormone replacement therapy / iron salts / MAOI antidepressants / methyldopa / perborates / peroxides / phenothiazines / proton pump inhibitors (e.g., omeprazole, lansoprazole) / tricyclic antidepressants.”

The commonest predisposing factor comes down to those little bumps on the tongue known as papillae.

According to the same source as above …

“The filiform papillae are excessively long (normal papillae are < 1 mm in length), and stained by the accumulation of epithelial squames, food debris and chromogenic micro-organisms. The dorsum of tongue is the main oral reservoir of micro-organisms such as Candida albicans and viridans streptococci, and those implicated in oral malodor." Peter A. Reichart & Hans Peter Philipsen (Oral Pathology, 2010) wrote … "In patients with hairy tongue, the filiform papillae of the dorsum of the tongue are excessively long. This is due to a delay in the normal process of epithelial desquamation or an increase in epithelial keratin formation. Individual filiform papillae can become 15-20 mm long and 2 mm thick. The color of hairy tongue varies from white to yellow or from brown to black. The dark pigmentation results from an increase in the number of chromatic microorganisms." Underproducing keratin causes weak nails and hair. Overproducing keratin is involved in black hairy tongue. Calcium, magnesium, vitamin B5 (pantothenic acid), vitamin B6 (in one or more of its seven vitamers), coenzyme A, and the sulfur-containing amino acid cysteine are involved in one way or another. Copper doesn't cause "black hairy tongue," but a lack of copper causes "green hairy tongue." A green hairy tongue is a symptom of copper deficiency myeloneuropathy. Acquired copper deficiency is rare, but increasing because of zinc and/or resveratrol abuse, especially resveratrol products bound to SLNs (solid lipid nanoparticles) and NLCs (nanostructured lipid carriers) to jack up its "poor bioavailability, low water solubility, and chemical instability." Many mainstream junk scientists and supplement sellers never got the memo that millions of food chemicals are poorly absorbed and assimilated to protect us from overdosing as well as from acute and chronic poisoning. Would you like to know more information about reseveratrol? Seek out an unbiased viticulturist or oenologist, not a biased salesman. (To Be Continued) . .