Famotidine

Histamine H2-receptor antagonist Gastrointestinal agent

Dosage & Administration Guidelines

Species Route Dose Frequency Duration & Clinical Notes
Dog PO 0.5-1 mg/kg q12h Duration: 2-8 weeks depending on condition
Notes: For mast cell tumors, use 1 mg/kg q12h. For severe ulcers, may use 1 mg/kg q12h.
Dog IV 0.5-1 mg/kg q12h Duration: Until oral administration is possible
Notes: Administer slowly over 2 minutes.
Cat PO 0.5-1 mg/kg q12h Duration: 2-8 weeks depending on condition
Notes: For mast cell tumors, use 1 mg/kg q12h.
Cat IV 0.5-1 mg/kg q12h Duration: Until oral administration is possible
Notes: Administer slowly over 2 minutes.
Horse PO 2-4 mg/kg q24h Duration: 4-6 weeks for ulcer healing
Notes: May be given once daily at night for better acid suppression.
Horse IV 0.3-0.5 mg/kg q12h Duration: Until oral administration is possible
Notes: For acute management.
Cattle PO 0.5-1 mg/kg q12h Duration: 5-7 days
Notes: Use cautiously in food animals; observe withdrawal times.
Small Ruminants PO 0.5-1 mg/kg q12h Duration: 5-7 days
Notes: Use cautiously; observe withdrawal times.
Rabbit PO 0.5-1 mg/kg q12h Duration: 5-7 days
Notes: May be used as adjunct in gastric stasis.
Bird/Poultry PO 0.5-1 mg/kg q12h Duration: 5-7 days
Notes: Limited studies; use with caution.

Clinical Indications & Species Uses

General Indications
  • Reduction of gastric acid secretion in conditions where acid suppression is beneficial
Dog (Canine)
  • Treatment of gastric and duodenal ulcers
  • Management of gastritis
  • Reduction of gastric acid secretion in esophagitis
  • Adjunct in treatment of mast cell tumors (to reduce histamine-induced acid secretion)
  • Prevention of stress ulcers
  • Treatment of Helicobacter-associated gastritis (in combination with antibiotics)
Cat (Feline)
  • Treatment of gastric ulcers
  • Management of gastritis
  • Reduction of gastric acid secretion in esophagitis
  • Adjunct in treatment of mast cell tumors
  • Prevention of stress ulcers
Horse (Equine)
  • Treatment of gastric ulcers (equine gastric ulcer syndrome)
  • Prevention of gastric ulcers in performance horses
Cattle (Bovine)
  • Treatment of abomasal ulcers
  • Management of abomasal bloat (as adjunct)
Small Ruminants (Sheep / Goat)
  • Treatment of abomasal ulcers
  • Management of gastritis
Rabbit & Small Mammals
  • Treatment of gastric ulcers
  • Management of gastric stasis (as adjunct)
Avian & Poultry
  • Treatment of gastric ulcers
  • Management of regurgitation and gastrointestinal irritation
Exotic & Other Species
  • Treatment of gastric ulcers in ferrets, reptiles, and other exotic species (off-label)

Pharmacology & Mechanism of Action

Drug Class: Histamine H2-receptor antagonist | Pharmacological Group: Gastrointestinal agent

Mechanism of Action: Famotidine is a competitive, reversible inhibitor of histamine at the H2-receptor on gastric parietal cells. By blocking histamine-induced acid secretion, it reduces basal and stimulated gastric acid production, including that stimulated by food, pentagastrin, and insulin. It also decreases pepsin output and gastric juice volume.

Pharmacodynamics: Famotidine produces a dose-dependent inhibition of gastric acid secretion. It is more potent than cimetidine and ranitidine on a molar basis. It does not affect gastric emptying, lower esophageal sphincter pressure, or pancreatic secretion. It has minimal antiandrogenic effects and does not inhibit cytochrome P450 enzymes significantly.

⚡ Pharmacokinetics Summary

Absorption: Rapidly absorbed after oral administration in most species. Bioavailability is approximately 40-50% in dogs and cats, with peak plasma concentrations occurring 1-2 hours after dosing. Food may slightly reduce absorption.
Distribution: Widely distributed in body tissues and fluids. It crosses the placenta and is excreted in milk. Protein binding is low (15-20%).
Metabolism: Minimal hepatic metabolism; primarily excreted unchanged in urine. Some metabolism occurs in the liver to inactive metabolites.
Excretion: Excreted primarily by the kidneys via glomerular filtration and tubular secretion. In renal impairment, dosage adjustment may be necessary.
Half-Life: Dogs: approximately 2-4 hours; Cats: approximately 2-3 hours; Horses: approximately 2-3 hours; Humans: 2.5-3.5 hours.
Bioavailability: Oral: 40-50% in dogs and cats; variable in other species.
Protein Binding: 15-20%

Available Formulations & Strengths

Oral Tablet 10 mg, 20 mg, 40 mg (PO)
Oral Suspension 40 mg/5 mL (PO)
Injectable Solution 10 mg/mL (IV)

Contraindications & Clinical Warnings

Contraindications:
  • Hypersensitivity to famotidine or other H2 antagonists
  • Severe renal impairment (may require dose adjustment)
  • Use in animals with known hepatic encephalopathy (may worsen condition)
Warnings & Clinical Precautions:
  • Use with caution in animals with renal or hepatic disease; dose adjustment may be necessary.
  • May mask symptoms of gastric cancer; rule out malignancy before treatment.
  • Use in pregnant or lactating animals only if clearly needed; safety not fully established.
  • In food animals, observe withdrawal times.
  • Long-term use may lead to vitamin B12 deficiency due to reduced acid secretion.
  • May increase risk of enteric infections due to reduced gastric acidity.

Adverse Effects & Reactions

Common:

  • Vomiting
  • Diarrhea
  • Constipation
  • Anorexia
  • Headache (in humans)

Serious / Severe:

  • Arrhythmias (rare)
  • Blood dyscrasias (rare)
  • Hepatotoxicity (rare)
  • Interstitial nephritis (rare)

Rare:

  • Confusion
  • Hallucinations
  • Seizures
  • Gynecomastia
  • Impotence

Key Drug Interactions

Interacting Drug / Class Clinical Effect & Mechanism Severity
Ketoconazole Reduced absorption of ketoconazole due to increased gastric pH; separate dosing by at least 2 hours. Moderate
Itraconazole Reduced absorption of itraconazole; separate dosing. Moderate
Antacids May reduce absorption of famotidine; separate administration by 1-2 hours. Mild
Sucralfate May reduce absorption of famotidine; separate administration by at least 2 hours. Mild
Cefpodoxime Reduced absorption of cefpodoxime due to increased gastric pH; monitor efficacy. Moderate
Iron supplements Reduced absorption of iron due to increased gastric pH; separate dosing. Mild

Overdose & Toxicity Management

Signs of Toxicity:

  • Vomiting
  • Diarrhea
  • Lethargy
  • Tachycardia
  • Hypotension
  • Respiratory depression (in severe cases)

Emergency Treatment Protocol: Treatment is symptomatic and supportive. Induce emesis if recent ingestion and animal is conscious. Administer activated charcoal to reduce absorption. Provide IV fluids for hypotension. Monitor ECG and respiratory status. There is no specific antidote.

Food Animal Withdrawal Times

🥩 Meat: 0 days🥛 Milk: 0 days🥚 Eggs: 0 days

Famotidine is not approved for food animals in many countries; however, if used extra-label, a withdrawal time of at least 24 hours for meat and milk is recommended. Consult regulatory guidelines.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature 20-25°C (68-77°F), excursions permitted between 15-30°C (59-86°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.

Extra-Label (Off-Label) Use: In the US, famotidine is not FDA-approved for veterinary species; use in animals is extra-label. Under AMDUCA, extra-label use is permitted by or on the order of a veterinarian within a valid VCPR, provided that withdrawal times are established and observed.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Famotidine is a potent H2 antagonist commonly used in veterinary medicine for acid-related gastrointestinal disorders. It is generally well tolerated, but its efficacy in raising gastric pH may be less than proton pump inhibitors (e.g., omeprazole) in some species, especially horses. For severe ulcers or esophagitis, consider using a PPI. In dogs and cats, famotidine is often used as an adjunct in mast cell tumor therapy to prevent histamine-induced gastric acid secretion. Dosing frequency may need adjustment in renal impairment. In food animals, use is extra-label and withdrawal times must be observed. Always consider the underlying cause of gastrointestinal signs and rule out neoplasia or foreign bodies.

References & Literature

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