Question: Can Pepto-Bismol or Kaopectate Turn Your Stool Black?
A small amount of a substance called sulfur is present in saliva and the gastrointestinal tract. Sulfur interacts with bismuth, the active ingredient in Pepto-Bismol. The result is bismuth sulfide, which causes the stool to turn black.
The black or green stool can last for several days, but it is harmless. This change in stool color is not a cause for concern unless you have stopped taking the bismuth subsalicylate and the unusual stool color persists. If your stool continues to be black or green, or is accompanied by diarrhea, vomiting, abdominal pain or other symptoms, talk to your physician.
American Society of Health-System Pharmacists, Inc. "Bismuth Subsalicylate." MedlinePlus. 1 Feb 2011. 13 Sept 2013.