...what is my poop trying to tell me? http://gizmodo.com/what-your-poop-is-trying-to-tell-you-1564158256 Black: Black stool is a surprisingly common side effect and can be caused from anything from a night of binging on black licorice and Guinness, to your iron supplement regimen, to ingesting large amounts of bismuth subsalicylate aka Pepto-Bismol. This happens when the bismuth subsalicylate combines with trace amounts of sulphur in your saliva to form bismuth sulfide, a highly insoluble black salt that can stain the tongue and stool jet black. Luckily, it is a temporary condition. However, black stool may also be an indicator of bleeding in the upper gastrointestinal tract, caused potentially by an ulcer or tumor. This bloody stool will often take on a tar-like appearance and smell horrible, so if you suddenly poop a poop that's black and sticky and smells like something crawled up there to die but haven't spent the last 16 hours at the pub, see your doctor.