Drugs Affecting GI Tract Motility: Opioids as Antidiarrheal Agents

Diarrhea results from increased gut motility and water and electrolyte secretion, causing frequent loose stools. Viral or bacterial infections, food poisoning, anxiety, radiation, or drug therapy can trigger it. Severe diarrhea leads to dehydration, electrolyte imbalances, and malnutrition if untreated. Symptoms include abdominal pain, bloating, nausea, and cramping.

Opioids bind to μ- or δ-opioid receptors in the gastrointestinal tract, reducing intestinal motility and enhancing fluid absorption. This mechanism slows bowel movements and decreases water and electrolyte secretion. Common opioid antidiarrheal agents include loperamide, diphenoxylate, and difenoxin, which are effective for managing acute diarrhea.

Loperamide shows poor blood-brain barrier penetration, resulting in minimal central nervous system effects compared to other opioids. It acts rapidly after oral intake by increasing anal sphincter tone and delaying fecal transit. Available as solutions, capsules, or chewable tablets, loperamide provides effective diarrhea relief without significant systemic opioid side effects.

Overdosing on opioids like loperamide and diphenoxylate can cause constipation, central nervous system depression, and paralytic ileus. Excessive diphenoxylate intake may result in severe colonic inflammation known as toxic megacolon. These serious complications require immediate medical attention and careful dose monitoring during treatment.

Diphenoxylate undergoes de-esterification to form difenoxin, an active metabolite that enhances antidiarrheal effects. It is indicated for moderate to severe diarrhea and is often combined with atropine to discourage misuse. However, atropine causes anticholinergic effects like constipation, nausea, dry mouth, and blurred vision, distinguishing it from loperamide.

Racecadotril is a prodrug converted to thiorphan, an enkephalinase inhibitor that increases endogenous opioid enkephalin levels by inhibiting their degradation. This action reduces fluid secretion and enhances intestinal absorption without blood-brain barrier penetration. Safe for acute diarrhea in children, racecadotril is not approved in the United States.

Loperamide should be discontinued if acute diarrhea does not reverse within two days of treatment. It is contraindicated in children below two years of age due to safety concerns. Patients should also avoid loperamide if they have symptoms suggesting infectious diarrhea or conditions requiring medical evaluation.