Black discoloration on the tongue could be from several causes: Some include medications that have bismuth ( like Peptobismol) or other antacids.
This is more common in smokers, or poor oral hygiene, but can occur for no known reason. It can be from certain antibiotics, radiation, and sometimes associated with fungal infections of the mouth.
The most common cause is just "idiopathic"- for no known reason there is just extra build up of keratin in the papillae- it has no medical significance at all in this case.
It is not suggestive of an STD, although it is more commonly seen in HIV infected men.
treatment involves optimizing oral hygiene, a tongue scraper, and antifungals if this is thought to be a fungal infection.
The "strawberry " tongue as well, if you have had it for many years is not likely to be an acute infection, but related to you individual papillae.
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