Pepto-Bismol is famous for turning your tongue and your stool black. It is not a sign of internal bleeding. The black color comes from a chemical reaction between the active ingredient, bismuth subsalicylate, and sulfur in your digestive tract. This reaction forms bismuth sulfide, a black compound that is harmless and simply passes through your system.
What Causes the Black Stool from Pepto-Bismol?
The active ingredient in Pepto-Bismol is bismuth subsalicylate. When you swallow it, the bismuth combines with the sulfur compounds naturally present in your saliva and your digestive tract. This chemical reaction creates bismuth sulfide.
Bismuth sulfide is a black, insoluble salt. Because your body cannot break it down or absorb it, it travels through your intestines and is eliminated in your stool. The result is a stool that can range from dark green to pitch black.
This is the same reason your tongue can turn black or dark gray after taking the medicine. The reaction happens in the mouth too, where saliva contains sulfur. The discoloration is temporary and fades as the medication clears your system.
The effect is completely harmless. The black stool is not a sign of damage, bleeding, or an allergic reaction. It is a direct, predictable result of the medication’s chemistry.
How Long Does the Black Stool Last?
The black color usually appears within a few hours of taking the first dose. It can persist for several days after you stop taking the medication.
Bismuth subsalicylate has a half-life of roughly 21 days in the body. This means it takes about three weeks for half of the medication to be eliminated. In practice, most people see their stool return to a normal color within 2 to 3 days after their last dose. For some people it may take up to a week.
If your stool remains black for more than a few days after stopping Pepto-Bismol, contact a healthcare provider. The medication should be out of your system by then, and other causes of black stool should be considered.
Black Stool from Pepto-Bismol vs. Black Stool from Bleeding
Black stool can also be a sign of gastrointestinal bleeding. Distinguishing between the two is important.
Pepto-Bismol produces a stool that is often described as dark gray or black with a pasty consistency. It typically has no strong odor.
Black stool from bleeding, called melena, is usually sticky, tar-like, and has a distinctly foul smell. This is because the blood has been digested by enzymes in the intestines. Melena often indicates bleeding in the upper digestive tract, such as the stomach or the first part of the small intestine.
If you are taking Pepto-Bismol and have any doubt about the cause of your black stool, consider the following signs that point toward bleeding:
- The stool is sticky and tar-like rather than pasty
- The stool has a strong, foul odor
- You feel dizzy, lightheaded, or unusually tired
- You have abdominal pain that is getting worse
- You vomit blood or material that looks like coffee grounds
If you experience any of these symptoms, seek medical care promptly. Do not assume the black stool is from the medication.
Other Medications and Foods That Can Darken Stool
Pepto-Bismol is not the only substance that can change stool color. Several common medications and foods have similar effects.
Iron supplements are a frequent cause of dark, greenish-black stool. This is normal and harmless, though it can be alarming if you are not expecting it.
Activated charcoal, sometimes used for gas relief or in emergency settings for poisoning, also produces black stool. The charcoal itself is not absorbed and passes through the digestive tract unchanged.
Bismuth subsalicylate is also found in other products, including Kaopectate and generic versions of Pepto-Bismol. Any product containing bismuth subsalicylate can cause black tongue and black stool.
Certain foods can darken stool as well:
- Black licorice
- Blueberries
- Dark chocolate
- Red beets (which can make stool appear red or black)
If you are not taking Pepto-Bismol or iron supplements and your stool turns black, do not assume it is from food. Black stool without an obvious cause should be evaluated by a healthcare provider.
Is It Safe to Keep Taking Pepto-Bismol?
Yes, it is safe to continue taking Pepto-Bismol according to the label directions even if your stool turns black. The discoloration is a known, expected side effect and not a reason to stop the medication.
However, bismuth subsalicylate contains a salicylate, which is chemically related to aspirin. Do not take Pepto-Bismol if you are allergic to aspirin or other salicylates. Do not give it to children or teenagers recovering from viral infections, particularly chickenpox or flu-like symptoms, because of the risk of Reye’s syndrome.
Pepto-Bismol is not recommended for long-term use. The label advises not to use it for more than 2 days for diarrhea unless directed by a doctor. Prolonged use can lead to bismuth toxicity, which is rare but can cause neurological symptoms.
If your diarrhea lasts more than 2 days, or if you have a fever, stop taking the medication and contact a healthcare provider. Persistent diarrhea can indicate an infection or another condition that needs different treatment.
When to See a Doctor About Black Stool
If you have been taking Pepto-Bismol and your stool is black, you generally do not need to see a doctor. The color change is expected and harmless.
See a doctor if:
- Your stool remains black more than 3 days after your last dose of Pepto-Bismol
- You were not taking Pepto-Bismol or iron supplements when the black stool appeared
- The stool is tar-like, sticky, or has a foul odor
- You have symptoms of anemia, such as fatigue, weakness, or shortness of breath
- You have unexplained weight loss or persistent abdominal pain
Black stool from bleeding is a medical condition that requires evaluation. Upper gastrointestinal bleeding can be caused by ulcers, gastritis, or other conditions. Prompt evaluation can identify the cause and prevent complications.
What About Black Tongue?
Pepto-Bismol can also turn your tongue black or dark gray. This is the same chemical reaction that causes black stool. Bismuth reacts with sulfur in saliva to form bismuth sulfide, which deposits on the tongue.
The discoloration is harmless and temporary. It typically resolves within a few days of stopping the medication. Brushing your tongue gently can help remove the discoloration faster, but the color will fade on its own.
Black tongue from Pepto-Bismol is not the same as “black hairy tongue,” a condition where the tongue’s papillae grow longer and trap bacteria and debris. That condition has different causes, including poor oral hygiene, tobacco use, and certain antibiotics. The discoloration from Pepto-Bismol is simply a coating on the tongue’s surface.
Frequently Asked Questions
How long after taking Pepto-Bismol does stool turn black?
Stool can turn black within a few hours of taking the first dose. The color typically lasts 2 to 3 days after your last dose.
Can Pepto-Bismol black stool be mistaken for blood in stool?
Yes, it can be mistaken for bleeding, but the two look different. Pepto-Bismol stool is pasty and has no strong odor, while bleeding produces sticky, tar-like stool with a foul smell.
Is black stool from Pepto-Bismol dangerous?
No, it is harmless. The black color comes from bismuth sulfide, a compound your body cannot absorb, so it passes through and colors the stool.
What should I do if my stool is black but I did not take Pepto-Bismol?
Contact a healthcare provider. Black stool without an obvious cause like Pepto-Bismol or iron supplements may indicate bleeding in the upper digestive tract.
