Can Pepto Bismol (bismuth subsalicylate) cause black stool?

Bismuth Subsalicylate Causes Black Stool

Yes, Pepto Bismol (bismuth subsalicylate) commonly causes black stools, which is a harmless side effect that occurs when bismuth combines with sulfur in the digestive tract. 1

Mechanism and Characteristics

• Bismuth subsalicylate causes a temporary darkening of the stool and/or tongue, which is harmless and expected when taking this medication 1
• In the gastrointestinal tract, bismuth subsalicylate is converted to salicylic acid and insoluble bismuth salts, which contribute to the black discoloration 2
• This discoloration occurs because bismuth combines with sulfur in the digestive tract to form bismuth sulfide, which has a black appearance 2

Clinical Significance

• The black stool caused by bismuth subsalicylate should be distinguished from melena (black, tarry stools caused by upper GI bleeding) 3
• Unlike melena, bismuth-related black stools:
  • Are not typically tarry in consistency
  • Do not have the characteristic foul odor of melena
  • Will resolve once the medication is discontinued 1, 2

Pharmacology and Safety

• Very little bismuth from bismuth subsalicylate is absorbed from the gastrointestinal tract (less than 0.005%) 2
• The salicylate portion is extensively absorbed (greater than 90%) and excreted in urine 2
• Even with extended dosing (3.14 g of BSS/day for up to 6 weeks), bismuth blood concentrations remain well below levels associated with neurotoxicity 2

Clinical Uses of Bismuth Subsalicylate

• Commonly used for treatment of mild travelers' diarrhea 3
• Can be used for prevention of travelers' diarrhea, with protection rates of 62-65% at higher doses 4
• Has antimicrobial, anti-inflammatory, and antisecretory properties 5, 6
• Component of quadruple therapy for Helicobacter pylori infections 7

Important Considerations

• Patients should be informed about this expected side effect to prevent unnecessary concern 1
• The black discoloration is temporary and will resolve after discontinuation of the medication 1
• Bismuth subsalicylate should be avoided in patients with aspirin allergy due to its salicylate content 2
• In clinical settings where actual melena is a concern, it may be necessary to temporarily discontinue bismuth products to accurately assess for GI bleeding 3

Dosing Considerations

• Standard dosing for diarrhea in adults is typically 262 mg tablets, with dosing frequency dependent on the indication 3
• The loperamide starting dose for mild travelers' diarrhea is 4 mg (2 tablets), followed by 2 mg after each loose stool (maximum 16 mg per day) 3
• For bismuth subsalicylate, doses of 2.1-4.2 g/day have been studied for prevention of travelers' diarrhea 4