Why does Pepto-Bismol turn your tongue black?

The change can appear within hours of the first dose. Typically the whole top of the tongue goes gray-black or jet black, and the surface stays smooth. That smoothness is a useful clue: in a published case review of bismuth-related black tongue, the authors noted that tongues darkened by medicines like this had a smooth surface without the enlarged papillae seen in black hairy tongue. In that case, the man's tongue returned to pink the same day, and taking the antacid again brought the color back, which confirmed the cause.

Many people also notice black or very dark stools at the same time. Both come from the same bismuth reaction, one in the mouth and one in the gut.

Bismuth is a metal. Your mouth and your colon both contain small amounts of hydrogen sulfide made by bacteria. When bismuth salts meet that sulfide, they form bismuth sulfide, which is insoluble and black. The StatPearls review of bismuth subsalicylate describes this reaction as the reason for both the dark tongue and the dark stool.

That means anything that makes the reaction more likely can make the tongue darker: a thick tongue coating that holds more bacteria and more medicine, chewable tablets that sit in the mouth, or taking a dose right before sleep when saliva flow slows. None of these are a problem in themselves; they just change how dark the stain gets.

This is the same family of products sold under store brands as "stomach relief" or "pink bismuth," so check the label for bismuth subsalicylate if you are not sure what you took.

The drug facts label for bismuth subsalicylate calls it "a temporary, but harmless, darkening of the stool and/or tongue,", and MedlinePlus says it usually goes away a few days after you stop the medicine. In practice the tongue usually looks better each time you eat, drink and brush, because the deposit sits on the surface.

To speed things up:

• Brush or scrape the tongue. The MSD Manual notes that bismuth discoloration on the top of the tongue can be removed by brushing or with a tongue scraper. Work back to front with light pressure and rinse between passes.
• Rinse with water after each dose if you still need to take the medicine. It reduces how much bismuth stays on the tongue.
• Swallow liquid doses promptly rather than swishing them around.
• Skip harsh fixes. Scrubbing hard, or gargling hydrogen peroxide, irritates the tongue and is not needed; peroxide rinses are themselves a trigger for hairy tongue.

If the tongue is still black a week after your last dose, something else is likely involved. Our main black tongue guide covers the other causes.

The two can overlap. Mayo Clinic notes that bismuth staining can look like black hairy tongue, and someone who already has a thick coating may see a darker, patchier result. For texture and cleaning tips specific to the furry version, see our hairy tongue scraping guide.

MedlinePlus lists bismuth medicines, iron tablets, black licorice and blueberries as harmless reasons for dark stool. It also warns that black, tarry, foul-smelling stool can mean bleeding in the esophagus, stomach or first part of the small intestine.

Because you may be taking Pepto for stomach symptoms in the first place, it is worth knowing the difference. Bismuth stool is dark gray to black but formed and normal-smelling. Stool that is sticky and tar-like, especially with dizziness, weakness, vomiting blood or vomit that looks like coffee grounds, needs medical attention right away, whether or not you took Pepto.

• It contains a salicylate. Bismuth subsalicylate is chemically related to aspirin. MedlinePlus advises telling your doctor or pharmacist if you are allergic to aspirin or similar pain relievers, or take a daily aspirin.
• Not for young children. HealthyChildren.org (American Academy of Pediatrics) says not to give children stomach medicines with salicylates such as bismuth subsalicylate unless they are at least 12, because salicylates are linked to Reye's syndrome in children with viral illnesses like flu and chickenpox. The drug facts label adds that children and teenagers who have or are recovering from chickenpox or flu-like symptoms should not use it.
• Short-term use only. MedlinePlus says to stop and contact a doctor if diarrhea lasts longer than 48 hours or symptoms get worse, and to stop immediately if you get ringing or buzzing in the ears.