As an antidiarrheal, the exact mechanism has not been determined. Bismuth subsalicylate may exert its antidiarrheal action not only by stimulating absorption of fluid and electrolytes across the intestinal wall (antisecretory action) but also, when hydrolyzed to salicylic acid, by inhibiting synthesis of a prostaglandin responsible for intestinal inflammation and hypermotility. In addition, bismuth subsalicylate binds toxins produced by Escherichia coli. Both bismuth subsalicylate and the intestinal reaction products, bismuth oxychloride and bismuth hydroxide, are believed to have bactericidal action. As an antacid, bismuth has weak antacid properties.
Bismuth subsalicylate displays anti-inflammatory action (due to salicylic acid) and also acts as an antacid and mild antibiotic. It can also cause a black tongue and black stools in some users of the drug, when it combines with trace amounts of sulfur in their saliva and gastrointestinal tract. This discoloration is temporary and harmless.
Based on in vitro dissociation data and in vivo animal data, bismuth subsalicylate is believed to be largely hydrolyzed in the stomach to bismuth oxychloride and salicylic acid. In the small intestine, nondissociated bismuth subsalicylate reacts with other anions (bicarbonate and phosphate) to form insoluble bismuth salts. In the colon, nondissociated bismuth subsalicylate and other bismuth salts react with hydrogen sulfide to produce bismuth sulfide, a highly insoluble black salt responsible for the darkening of the stools.
Goldenberg MM, Honkomp LJ, Burrous SE, Castellion AW: Protective effect of Pepto-Bismol liquid on the gastric mucosa of rats. Gastroenterology. 1975 Sep;69(3):636-40. Pubmed