Bismuth subsalicylate - Black tongue! Black poop!

Bismuth subsalicylate is the active ingredient in Pepto-Bismol (or if you're a cheapo university student like me, Life Brand Stomach Ache Settler) and Kaopectate. Incidentally, Pepto-Bismol ice cream is a thing.

Consisting of bismuth (a cool-looking metal that like water is less dense as a solid than a liquid) and salicylic acid (essentially the same drug found in aspirin), bismuth subsalicylate has arguably one of the best harmless side effect of all time: temporary darkening of the tongue and/or stool. The bismuth in the drug can react with tiny amounts of sulfur found in saliva and the gastrointestinal tract to form bismuth sulfide, which like many sulfide minerals (e.g. ferrous sulphide, lead sulphide), is black.

It is purported to be an excellent hangover cure, which makes sense given its stomach-settling properties. In addition to its painkilling effect, salicylic acid may be active against stomach-upsetting bacteria. As a sort of paradox, salicylic acid, when taken chronically, can cause peptic ulcers, while bismuth salicylate, which contains salicylic acid, can be used to prevent and treat such ulcers (by coating the lining of the stomach to help protect it from digestive juices).