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Drugs for Treatment of Diarrhea

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1. Opioid Antidiarrheal Agents (Antimotility)

Loperamide (drug of choice for most acute diarrhea)

• Mechanism: MOR agonist; 40–50× more potent than morphine as an antidiarrheal. Increases small intestinal transit time, increases anal sphincter tone, and has antisecretory activity against cholera toxin and E. coli toxin. Penetrates CNS poorly — no significant abuse potential at therapeutic doses.
• Dose: 4 mg initially, then 2 mg after each loose stool; max 16 mg/day in adults.
• Cautions: FDA black box warning — exceeding recommended dose can cause torsades de pointes, cardiac arrest, death. Avoid or use with extreme caution in acute IBD colitis (risk of toxic megacolon). Not for children < 2 years.
• Goodman & Gilman's, Sleisenger & Fordtran's

Diphenoxylate/Atropine (Lomotil) & Difenoxin/Atropine (Motofen)

• Mechanism: Meperidine-related opioids; schedule V controlled substances; co-formulated with subtherapeutic atropine to discourage abuse.
• Dose: Two tablets initially, then 1 tablet every 3–4 h; max diphenoxylate 20 mg/day, difenoxin 8 mg/day.
• Cautions: CNS depression in high doses; anticholinergic effects (dry mouth, blurred vision); toxic megacolon in colitis.
• Goodman & Gilman's

Codeine, Tincture of Opium, Paregoric, Morphine

• Used for chronic diarrhea refractory to loperamide (e.g., secretory diarrhea, post-surgical).
• Codeine: 15–60 mg 4× daily; morphine 2–20 mg 4× daily.
• Deodorized tincture of opium (0.6 mL = 6 mg morphine) is 25× stronger than paregoric — dangerous confusion between the two formulations can cause overdose.
• Tolerance to GI effects does not develop, so dose remains stable over time.
• Sleisenger & Fordtran's, Goodman & Gilman's

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2. Enkephalinase Inhibitor (Antisecretory)

Racecadotril (Acetorphan)

• Mechanism: Prodrug → thiorphan, which inhibits neutral endopeptidase (enkephalinase), thereby increasing endogenous enkephalin levels → DOR (delta opioid receptor) activation → antisecretory effect. Acts peripherally; does not cross CNS.
• Dose: 100 mg orally every 8 h up to 7 days; in children 1.5 mg/kg every 8 h with ORS.
• Advantage over loperamide: Less constipation; well-tolerated in children.
• Note: Available in many countries, but not approved in the USA.
• Goodman & Gilman's, Sleisenger & Fordtran's

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3. Bismuth Subsalicylate

• Brand names: Pepto-Bismol, Kaopectate
• Mechanism: Antisecretory, anti-inflammatory, and antimicrobial effects. The salicylate component is absorbed; bismuth passes mostly into feces.
• Uses: Traveler's diarrhea (prevention and treatment), acute gastroenteritis.
• Dose: 30 mL or 2 tablets every 30–60 min; up to 8 doses/day.
• Adverse effects: Black/tarry stools and black tongue (bismuth sulfide — not melena); tinnitus (salicylate toxicity at high doses); Reye's syndrome risk in children/teenagers with viral illness.
• Contraindications: Aspirin allergy, bleeding disorders, peptic ulcer disease, concurrent salicylate use.
• Goodman & Gilman's, Katzung's

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4. Adsorbents and Bulk-Forming Agents

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5. α₂-Adrenergic Agonist

Clonidine

• Mechanism: Stimulates α₂ receptors on enteric neurons and enterocytes → stimulates fluid/electrolyte absorption, inhibits secretion, increases transit time.
• Special role: Particularly useful in diabetic diarrhea and diarrhea from opioid withdrawal.
• Dose: 0.1–0.3 mg orally 3× daily; topical patch may provide steadier levels.
• Limitation: Hypotension, depression, and fatigue are dose-limiting.
• Goodman & Gilman's, Sleisenger & Fordtran's

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6. Somatostatin Analogue

Octreotide

• Mechanism: Inhibits secretion of 5-HT and GI peptides; reduces intestinal secretion and motility.
• Key indications: Diarrhea from carcinoid syndrome, VIPoma, glucagonoma, dumping syndrome, chemotherapy-induced diarrhea, AIDS-related diarrhea.
• Dose: 50–100 μg SC 2–3×/day, titrated up to 500 μg 3×/day. Long-acting depot (Sandostatin LAR) available monthly.
• Adverse effects: Cholelithiasis (bile stasis with long-term use), steatorrhea, nausea, hyperglycemia.
• Goodman & Gilman's, Sleisenger & Fordtran's

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7. Tryptophan Hydroxylase Inhibitor

Telotristat Ethyl (Xermelo)

• Mechanism: Inhibits tryptophan hydroxylase (rate-limiting enzyme of serotonin synthesis) → reduces 5-HT secretion → decreases fluid secretion and GI motility.
• Indication: Diarrhea of carcinoid syndrome inadequately controlled by somatostatin analogues alone.
• Dose: 250 mg orally 3× daily (with food).
• Adverse effects: Constipation, nausea, elevated GGT, depression.
• Goodman & Gilman's

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8. Bile Acid-Binding Resins

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9. Chloride Channel Inhibitor

Crofelemer (Mytesi)

• Mechanism: Inhibits CFTR (cystic fibrosis transmembrane regulator) and calcium-activated chloride channels on enterocytes → reduces chloride-driven secretory diarrhea.
• FDA-approved for: Non-infectious diarrhea in HIV/AIDS patients on antiretroviral therapy.
• Sleisenger & Fordtran's

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10. 5-HT₃ Receptor Antagonist

Alosetron (Lotronex)

• Indication: Diarrhea-predominant IBS (IBS-D) in women who have failed conventional therapy.
• Mechanism: Blocks 5-HT₃ receptors on enteric neurons → slows colonic transit, reduces visceral sensitivity.
• Risk: Ischemic colitis — restricted prescribing program in the USA.
• Goodman & Gilman's

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11. Antibiotics (for Infectious Diarrhea)

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12. Probiotics and Other Agents

• Probiotics (Lactobacillus GG, Saccharomyces boulardii): Some evidence in antibiotic-associated diarrhea, acute infectious diarrhea, and traveler's diarrhea. Evidence in chronic diarrhea remains limited.
• Berberine (plant alkaloid): Antimicrobial, antisecretory, inhibits smooth muscle contraction; used for bacterial diarrhea and cholera in some countries; not FDA-approved.
• Oral rehydration solution (ORS): Not a drug per se, but is the cornerstone of management for acute diarrhea to prevent/treat dehydration.

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Quick Reference Summary

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• Goodman & Gilman's The Pharmacological Basis of Therapeutics — Antimotility and Antisecretory Agents
• Sleisenger and Fordtran's Gastrointestinal and Liver Disease — Table 16.5: Nonspecific Drug Therapy for Chronic Diarrhea
• Harrison's Principles of Internal Medicine 22E (2025) — Pharmacotherapy section
• Katzung's Basic and Clinical Pharmacology, 16th Ed. — OTC Antidiarrheal Agents table