Yes, bismuth causes black stools. It’s one of the most common side effects of taking bismuth subsalicylate, the active ingredient in Pepto-Bismol and Kaopectate. In one clinical study, 80% of participants who took bismuth subsalicylate experienced darkened stools. The color change is harmless and temporary, but it can be alarming if you’re not expecting it, especially because black stools can also signal bleeding in the digestive tract.
Why Bismuth Turns Your Stool Black
The color change comes from a simple chemical reaction in your gut. Bacteria in your digestive tract naturally produce small amounts of sulfur compounds. When bismuth meets those bacterial sulfides, it forms a new compound called bismuth sulfide, which is intensely dark. This reaction happens throughout your intestines, so by the time stool reaches the end of its journey, it can look jet black.
The same reaction can also darken your tongue. Bismuth sulfide forms on the tongue’s surface when you chew or swallow the medication, creating a black or grayish-brown coating. In documented cases, this tongue discoloration has appeared anywhere from less than a day to several days after first taking bismuth. The color returns to normal once you stop taking it.
How Long the Color Change Lasts
Black stools typically persist for as long as you’re taking the medication and for a few days afterward, since it takes time for your body to clear the remaining bismuth sulfide. If you stop taking bismuth and your stools return to their normal brown color within a couple of days, there’s nothing to worry about. The discoloration is purely cosmetic and doesn’t indicate any damage to your digestive system.
Bismuth Black Stools vs. Bleeding
The real concern with black stools isn’t bismuth. It’s a condition called melena, which happens when blood from the stomach or upper intestines is digested as it moves through the gut, turning the stool dark. Both bismuth-related stools and melena can look jet black, so color alone isn’t enough to tell them apart.
The key differences are texture and smell. Stools caused by gastrointestinal bleeding tend to be sticky and tarry, with a distinctly foul odor that’s noticeably different from a normal bowel movement. Bismuth-darkened stools, by contrast, have a normal consistency and don’t carry that unusual smell. If your black stools are sticky, tarry, or accompanied by other symptoms like dizziness, fatigue, or stomach pain, that pattern points toward bleeding rather than a harmless medication side effect.
Bismuth Can Interfere With Stool Tests
If your doctor has ordered a fecal occult blood test (the guaiac card test used to screen for hidden blood in stool), bismuth can actually reduce the test’s sensitivity. Research has confirmed that Pepto-Bismol is among several substances that can cause false-negative results, meaning the test might miss blood that’s actually present. If you’re scheduled for stool testing, let your doctor know you’ve been taking bismuth so they can advise you on when to stop before the test.
Other Things That Turn Stool Black
Bismuth isn’t the only non-dangerous cause of black stools. Iron supplements are another common culprit, as are foods like black licorice and blueberries. If you’ve recently started any new supplement or changed your diet, that may explain the color shift without any cause for concern.
The important distinction is always whether you can trace the color change to something you consumed. If you took Pepto-Bismol yesterday and your stool is black today, the connection is straightforward. If you haven’t taken bismuth, iron, or any dark-colored foods, black stools deserve prompt medical attention because upper gastrointestinal bleeding is the most serious explanation.
Safety Considerations With Bismuth
While the stool color change itself is harmless, bismuth subsalicylate does contain salicylate, the same class of compound found in aspirin. Taking too much can cause symptoms of salicylate toxicity: ringing in the ears, hearing changes, confusion, rapid breathing, and severe drowsiness. Older adults are more sensitive to these effects.
You should also be cautious about combining bismuth with aspirin or other salicylate-containing medications, since the doses can add up. Stick to the recommended amount on the package, and be aware that black stools are a normal signal that the bismuth is moving through your system, not a sign you’ve taken too much.