Though there are medications for diarrhea available only by prescription, most cases of diarrhea can be treated successfully with over-the-counter products. But as with any medication, you always have to be aware of possible drug interactions.
There are two broad types of over-the-counter antidiarrheal medications: those that have loperamide as their primary ingredient, and those that have bismuth subsalicylate as their primary ingredient.
Loperamide-based antidiarrheals work by slowing down the movement of fluid through the intestines. These antidiarrheals are recommended for nonspecific acute diarrhea and for chronic diarrhea. Products sold under the brand name Imodium are loperamide antidiarrheals.
Bismuth subsalicylate-based antidiarrheals work by decreasing the secretion of fluid into the intestines, reducing inflammation, and stifling the growth of certain bacteria and viruses that can cause diarrhea. These antidiarrheals are recommended for mild nonspecific diarrhea. Products sold under the brand names Kaopectate and Pepto-Bismol are bismuth subsalicylate antidiarrheals.
Loperamide and bismuth subsalicylate antidiarrheals have different dosages, side effects, and warnings, so the directions for a given product should always be read and followed carefully.
Specifically, they also can give rise to different drug interactions.
Loperamide antidiarrheals can interact with antiviral medicines taken for the human immunodeficiency virus (HIV), antibiotics, and prescription pain medicines. Taking loperamide with any of these increases the risk of side effects, so anyone taking these medications should always consult with a doctor before taking loperamide.
Bismuth subsalicylate can interact with blood-thinning medicines, gout medicines, arthritis medicines, and diabetes medicines, increasing the risk of side effects. So here too, anyone taking these medications should always consult with a doctor before taking bismuth subsalicylate.
Also, ordinary aspirin is a salicylate. If you take a bismuth subsalicylate antidiarrheal along with any medicine that contains aspirin-including over-the-counter pain relievers and cold medicines-you risk consuming too much salicylate. Excessive salicylate can give rise to serious symptoms including swelling of the face, fingers, feet, and/or lower legs; rash; severe stomach pain; vomiting; or black tarry stools. Bismuth subsalicylate antidiarrheals should not be taken with any medicines that contain aspirin, unless directed by a doctor.
Nancy Ross-Flanigan, “Antidiarrheal Drugs.” Healthline.
Yvette C. Terrie, “A Pharmacist’s Guide to OTC Therapy: Antidiarrheal Products.” Pharmacy Times.
“Antidiarrheal Medicines: OTC Relief for Diarrhea.” Family Doctor.