Pepto is composed of pink bismuth, and even though it works, doctors aren't completely sure exactly why. They have some strong theories that make sense medically, but the exact mechanism isn't known. When you digest it, your body breaks it down to bismuth and other things. Since bismuth is essentially inert in the digestive system, it gets moved out as what it is: ultra-fine particles of a dull grey metal.

It makes poops turn grey or black, in some cases. Totally normal.