Loperamide Hydrochloride

Antidiarrheal Opioid agonist (peripheral)

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 0.1-0.2 mg/kg (up to 0.4 mg/kg) every 8-12 hours q8-12h Duration: Up to 3-5 days or until diarrhea resolves
Notes: Use with caution in dogs with hepatic dysfunction or in breeds with MDR1 mutation (e.g., Collies) – reduce dose or avoid.
Cat PO 0.04-0.08 mg/kg every 12 hours (max 0.1 mg/kg) q12h Duration: Short-term (1-2 days) only under veterinary supervision
Notes: Cats are sensitive to CNS effects; use extreme caution. Not recommended as first-line.
Horse PO Not recommended N/A Duration: N/A
Notes: Avoid in horses due to risk of colic and potential for worsening of infectious diarrhea.
Cattle PO Not recommended N/A Duration: N/A
Notes: Not approved for food animals; no withdrawal times established.
Small Ruminants PO Not recommended N/A Duration: N/A
Notes: Avoid use.
Rabbit PO Not recommended N/A Duration: N/A
Notes: Contraindicated due to risk of GI stasis.
Bird/Poultry PO Not recommended N/A Duration: N/A
Notes: Avoid use.
Exotic/Other PO Not recommended N/A Duration: N/A
Notes: Avoid use.

Clinical Indications & Species Uses

General Indications
  • Symptomatic relief of acute diarrhea in dogs when infectious causes have been ruled out or are being treated concurrently.
Dog (Canine)
  • Symptomatic treatment of acute non-specific diarrhea
  • Reduction of fecal output in chronic diarrhea (e.g., inflammatory bowel disease) as adjunctive therapy

Pharmacology & Mechanism of Action

Drug Class: Antidiarrheal | Pharmacological Group: Opioid agonist (peripheral)

Mechanism of Action: Loperamide is a synthetic opioid agonist that acts on μ-opioid receptors in the myenteric plexus of the gastrointestinal tract. It inhibits peristalsis by decreasing the release of acetylcholine and prostaglandins, thereby slowing intestinal motility and increasing transit time. It also increases anal sphincter tone and reduces fecal volume, fluid and electrolyte loss. At therapeutic doses, it does not cross the blood-brain barrier significantly, thus lacks central analgesic effects.

Pharmacodynamics: Loperamide binds to μ-opioid receptors in the gut, leading to reduced propulsive contractions and increased segmentation. It enhances absorption of water and electrolytes from the intestinal lumen. The drug has a rapid onset of action (within 1-2 hours) and its antidiarrheal effect lasts for 12-24 hours. It does not affect the CNS at normal doses due to P-glycoprotein efflux in the blood-brain barrier.

⚡ Pharmacokinetics Summary

Absorption: Loperamide is poorly absorbed from the gastrointestinal tract. Oral bioavailability is approximately 0.3% in humans, but may be higher in some animals. Peak plasma concentrations occur 2-5 hours after oral administration.
Distribution: Loperamide is extensively distributed to the intestinal wall, where it exerts its local effect. It has a large volume of distribution. It is a substrate for P-glycoprotein, which limits CNS penetration.
Metabolism: Loperamide is extensively metabolized in the liver via oxidative N-demethylation, primarily by CYP3A4 and CYP2C8. It undergoes significant first-pass metabolism.
Excretion: The drug is excreted primarily in the feces (about 97%) as unchanged drug and metabolites. Approximately 1-2% is excreted in urine. Biliary excretion is the major route for metabolites.
Half-Life: Half-life: Dogs: 2-4 hours; Cats: 2-3 hours; Humans: 9-14 hours.
Bioavailability: Oral bioavailability is low (0.3% in humans) due to extensive first-pass metabolism. In dogs, it may be slightly higher but still low.
Protein Binding: Approximately 95-97% bound to plasma proteins.

Available Formulations & Strengths

Oral Tablet 2 mg (PO)
Oral Capsule 2 mg (PO)
Oral Liquid 1 mg/5 mL (PO)

Contraindications & Clinical Warnings

Contraindications:
  • Known hypersensitivity to loperamide
  • Animals with known MDR1 gene mutation (e.g., Collies, Shelties, Australian Shepherds) – risk of CNS toxicity
  • Cats (especially kittens) – high risk of CNS excitation
  • Animals with infectious diarrhea (e.g., parvovirus, salmonellosis) – may worsen infection by delaying pathogen clearance
  • Animals with GI obstruction, ileus, or constipation
  • Animals with hepatic impairment (severe)
  • Animals with acute ulcerative colitis
  • Rabbits and other hindgut fermenters – risk of GI stasis
Warnings & Clinical Precautions:
  • Use with caution in dogs with hepatic or renal disease
  • Do not use in animals with high fever or systemic signs of infection
  • Monitor for signs of CNS depression or excitation, especially in cats and MDR1 mutant dogs
  • Use only for symptomatic relief; identify and treat underlying cause of diarrhea
  • Avoid in debilitated or geriatric animals
  • In food animals, do not use due to lack of withdrawal times and potential for residues
  • Keep out of reach of children; human formulations may contain other ingredients

Adverse Effects & Reactions

Common:

  • Constipation
  • Abdominal discomfort
  • Drowsiness (rare at therapeutic doses)

Serious / Severe:

  • CNS depression or excitation (especially in cats and MDR1 mutant dogs)
  • Paralytic ileus
  • Toxic megacolon (in colitis)
  • Respiratory depression (overdose)

Rare:

  • Allergic reactions (rash, urticaria)
  • Pancreatitis
  • Hepatic dysfunction

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
P-glycoprotein inhibitors (e.g., ketoconazole, itraconazole, verapamil, cyclosporine) Increased CNS penetration and toxicity of loperamide High
CYP3A4 inhibitors (e.g., erythromycin, clarithromycin, grapefruit juice) Increased plasma levels of loperamide, potential for toxicity Moderate
Opioid agonists (e.g., morphine, fentanyl) Additive CNS depression and constipation Moderate
Anticholinergic drugs (e.g., atropine) Additive anticholinergic effects (dry mouth, urinary retention, constipation) Moderate
Other antidiarrheal agents (e.g., bismuth subsalicylate) Potential for additive constipation Mild

Overdose & Toxicity Management

Signs of Toxicity:

  • CNS depression (lethargy, stupor, coma)
  • CNS excitation (in cats: agitation, tremors, seizures)
  • Respiratory depression
  • Paralytic ileus
  • Constipation
  • Mydriasis
  • Hypothermia

Emergency Treatment Protocol: Induce emesis if recent ingestion (within 1-2 hours) and animal is conscious. Administer activated charcoal to reduce absorption. Provide supportive care: IV fluids, respiratory support, and monitoring. Naloxone may be used to reverse CNS effects (0.01-0.04 mg/kg IV, titrated to effect). In severe cases, consider gastric lavage. Monitor for prolonged effects due to enterohepatic recirculation.

Food Animal Withdrawal Times

Not approved for use in food animals. No withdrawal times established. Do not use in food-producing animals.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F)

Handling & Special Conditions: Protect from moisture. Keep container tightly closed.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use (OTC).

Extra-Label (Off-Label) Use: In the US, loperamide is not FDA-approved for veterinary use; however, it may be used extra-label in dogs and cats under the Animal Medicinal Drug Use Clarification Act (AMDUCA) when a valid veterinarian-client-patient relationship exists. Extra-label use in food animals is prohibited.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Loperamide is a synthetic opioid used primarily in dogs for symptomatic treatment of acute diarrhea. It is not recommended in cats due to the risk of severe CNS toxicity, especially in kittens and those with MDR1 mutations. In dogs, it should be used with caution in breeds with MDR1 mutation (e.g., Collies) and in animals with infectious diarrhea, as it may delay clearance of pathogens. Always identify and treat the underlying cause of diarrhea. For chronic diarrhea, use only as adjunctive therapy. Monitor for adverse effects, particularly constipation and CNS signs. In food animals, loperamide is contraindicated due to lack of withdrawal data. It is available as OTC human products, but veterinary use should be under professional supervision.

References & Literature

  • 📚 Plumb's Veterinary Drug Handbook
  • 📚 Papich Veterinary Pharmacology and Therapeutics
  • 📚 Compendium of Veterinary Products (CVP)