Pepto Bismol can turn your poop black, and it often does. This is the most noticeable and alarming change, but it’s harmless. The drug can also firm up loose stools and reduce how often you need to go. Here’s what’s happening and how to tell if something else is going on.
Why Pepto Bismol Turns Your Poop Black
The active ingredient in Pepto Bismol is bismuth subsalicylate. When bismuth travels through your digestive system, it reacts with small amounts of sulfur naturally present in your saliva and gut. These two substances combine to form bismuth sulfide, a black-colored compound. That compound coats your stool as it moves through, giving it a dark or completely black appearance.
The same reaction can also darken your tongue, which catches people off guard. Both effects are cosmetic and temporary. They don’t mean anything is wrong with your digestive system.
Not everyone experiences it to the same degree. The amount of sulfur in your diet and gut plays a role. Foods high in sulfur, like eggs, garlic, broccoli, and meat, can make the darkening more pronounced.
How It Changes Stool Consistency and Frequency
Beyond color, Pepto Bismol changes how your stool looks and how often you go. The salicylate component of the drug reduces the amount of fluid your intestines secrete by calming down the inflammatory signals (prostaglandins) that ramp up during a bout of diarrhea. It also blocks certain signals from bacterial toxins that trigger watery stools.
The practical result: your stool becomes firmer and more formed, and you make fewer trips to the bathroom. If you’re taking it for an upset stomach rather than diarrhea, you may notice your stool is simply darker without much change in consistency.
How Long the Changes Last
Black stool typically continues for as long as you’re taking the medication and for a short period after you stop. Most people see their stool return to its normal color within a few days of their last dose, once the bismuth sulfide has fully cleared the digestive tract. The exact timeline depends on your individual gut transit time, which varies from person to person, but you shouldn’t still be seeing dark stool a week after stopping.
Black Stool vs. Signs of Bleeding
This is the important part. Black stool from Pepto Bismol looks different from black stool caused by bleeding in the upper digestive tract, a condition called melena. Knowing the difference matters.
- Texture: Melena is tarry and sticky, almost like thick tar or asphalt. Bismuth-stained stool has a more normal consistency and isn’t unusually sticky.
- Smell: Melena has a distinctly foul odor that’s noticeably worse than normal stool. That smell comes from blood being broken down as it travels through the digestive system. Bismuth-darkened stool doesn’t have this distinctive smell.
- Timing: If you haven’t taken Pepto Bismol or other bismuth products recently, black stool shouldn’t be dismissed as harmless.
If your stool is black, sticky, and unusually foul-smelling, or if you’re experiencing other symptoms like dizziness, weakness, or abdominal pain, that warrants medical attention regardless of whether you’ve taken Pepto Bismol. A simple stool test can determine whether blood is present.
Other Possible Color Changes
Black is the most common color shift, but bismuth subsalicylate can occasionally produce other variations. The Mayo Clinic lists it as a possible cause of light-colored or clay-colored stool, particularly at larger doses. Some people report dark green stool, which can happen when bismuth sulfide mixes with the normal green-brown pigments of bile as it moves through the intestines at different speeds.
A Note on Children and Salicylates
Pepto Bismol contains a salicylate, which is chemically related to aspirin. Children under 16 should not take salicylate-containing products because of the risk of Reye’s syndrome, a rare but serious condition that can develop when children with a viral illness are exposed to salicylates. This applies even when the concern is just an upset stomach. If your child has diarrhea, other options are safer.
How Long to Use It
If you’re taking Pepto Bismol for diarrhea and your symptoms haven’t improved within two days, or if they get worse, it’s time to check in with a doctor. The standard dosing allows up to 16 tablets or 16 tablespoons of regular-strength liquid in a 24-hour period, but most people don’t need anywhere near that amount. The medication is designed for short-term symptom relief, not ongoing daily use.