loperamide

Loperamide is a synthetic opioid medication used to treat diarrhea. It acts as an agonist at peripheral mu-opioid receptors in the enteric nervous system, which reduces intestinal motility and prolongs transit time to increase water absorption from stool. Because it is poorly able to cross the blood–brain barrier and is actively expelled from the brain by P-glycoprotein, central opioid effects are minimal at normal doses.

It is commonly used for short-term relief of acute nonspecific diarrhea and for control of chronic diarrhea in conditions such as inflammatory bowel disease or irritable bowel syndrome with diarrhea. Over-the-counter formulations for adults typically follow a dosing pattern of 2 mg after the first loose stool, then 1 mg after each subsequent loose stool, with a maximum of about 8 mg per day. Higher doses may be prescribed under medical supervision for certain chronic conditions, but use should be guided by a clinician. Loperamide should not be used for diarrhea caused by infections with fever, blood, or toxins, and symptoms persisting beyond 48 hours or accompanied by dehydration require medical attention. It is generally not recommended for young children; dosing in pediatric patients is more restricted and requires professional guidance.

Common side effects include constipation, abdominal cramps, nausea, and dizziness. Rare but serious risks include severe constipation, ileus, or toxic megacolon, particularly in susceptible individuals such as children. Loperamide can interact with drugs that affect P-glycoprotein or cardiac conduction, and excessive doses can cause cardiac arrhythmias or sudden death in rare cases. Because of potential misuse at high doses, some regulatory agencies have imposed restrictions on OTC sales and issued safety warnings.

Availability varies by country; loperamide is typically sold OTC in many regions, with prescription formulations used for higher-dose regimens under supervision.