I am on a antibiotic for lyme disease. i just noticed that my tongue is dark brown or black. why?

Antibiotic effect: Some antibiotics can cause a dark discoloration, appearing brown or black, or even "hairy." It will not hurt you. When you stop the antibiotic it will gradually go away.

Black hairy tongue: The finding is usually a result of taking antibiotics and leads to an overgrowth or dysbiosis of bacteria or yeast. Other causes include smoking or dry mouth. This will usually improve over time, and I don't always treat it.