Which of the following is an anti-diarrheal medication?

• A. Diphenoxylate
• B. Bismuth Subsalicylate
• C. Loperamide
• D. Paregoric

Answer: (C)

The idea being tested is how anti-diarrheal drugs work by slowing intestinal movement to give water and electrolytes a chance to be absorbed, reducing stool frequency and liquidity. Loperamide fits this by acting as a μ-opioid receptor agonist in the enteric nervous system, which decreases gut smooth muscle tone and slows motility, letting more water be reabsorbed. It achieves this with minimal CNS effects because it stays largely in the gut and is expelled from the brain by P-glycoprotein, giving effective relief from diarrhea without significant central opioid effects. While other options can have anti-diarrheal actions (diphenoxylate is another opioid antidiarrheal, paregoric is an opium tincture, and bismuth subsalicylate has antisecretory and protective effects), loperamide is the classic peripherally acting agent most commonly recognized for this purpose.