Bismuth Subsalicylate

• the active ingredient in Pepto-Bismol (or if you're a cheapo university student like me, Life brand Stomach Ache Settler) and Kaopectate
• is a salt consisting of bismuth (a cool-looking heavy metal) and salicylic acid (essentially the same drug found in aspirin)
• causes the best harmless side effect ever: temporary darkening of the tongue and/or stool (read: black poo!)
• purported to be an excellent hangover cure, which makes sense given its stomach-settling properties
• the crazy thing here is that 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)
  

http://en.wikipedia.org/wiki/Bismuth_subsalicylate