Famotidine
Famotidine is a potent, long-acting histamine H2-receptor antagonist used widely in veterinary medicine to suppress gastric acid production and treat or prevent gastric/duodenal ulcers, gastritis, and esophagitis. Compared to cimetidine, famotidine is significantly more potent, requires less frequent dosing, and crucially, **does not inhibit the hepatic cytochrome P450 enzyme system**, making it free of many common drug interactions. > **Clinical Pearl:** While effective for mild acid suppression or routine prophylaxis, proton pump inhibitors (PPIs) like omeprazole are generally considered superior for treating active ulcers, preventing exercise-induced gastritis, or managing severe esophagitis. Furthermore, famotidine is prone to pharmacological tolerance (**tachyphylaxis**) within 3 to 14 days of continuous use in dogs and cats, which diminishes its acid-suppressing efficacy over time.
Mechanism: Famotidine acts as a competitive, reversible antagonist at the **H2 receptors** located on the basolateral membrane of gastric **parietal cells**. By blocking histamine binding, it prevents the activation of adenylate cyclase โ decreases intracellular cAMP levels โ reduces the activation of the **H+/K+-ATPase proton pump**. This effectively blunts both basal gastric acid secretion and secretion stimulated by food, pentagastrin, or insulin. By decreasing the total volume of gastric juice produced, H2-blockers also indirectly decrease the amount of pepsin secreted. It does not alter gastric emptying time, biliary secretion, or lower esophageal sphincter pressure.
Dosing by species
- To reduce gastric acid production ยท 0.2 mg/kg ยท PO, SC, IM, IV ยท q24h ยท See warning about IV use in cats
- To reduce gastric acid production ยท 0.5 mg/kg ยท PO, SC, IM, IV ยท q12-24h
- For esophagitis ยท 0.5 mg/kg ยท PO ยท q12h
- For acute reflex esophagitis ยท 0.55-1.1 mg/kg ยท PO ยท q24h (or q12h if severe) ยท 2-3 weeks
- Adjunctive treatment of GI effects associated with chronic progressive renal disease ยท 1 mg/kg ยท PO ยท q24h
- Adjunctive treatment of GI effects associated with chronic progressive renal disease ยท 0.5-1 mg/kg ยท PO ยท q12-24h
- All uses (ulcers, gastritis, oesophagitis, hypersecretory conditions) ยท 0.5-1.0 mg/kg ยท PO ยท q12-24h ยท Effect on gastric pH diminishes with time when given daily.
- For stress induced ulcers ยท 0.25-0.5 mg/kg ยท PO, IV ยท q24h
- In combination with antibiotics for Helicobacter treatment ยท 0.25-0.5 mg/kg ยท PO, IV ยท q24h
- As an adjunct in ulcer treatment ยท 0.23 mg/kg or 0.35 mg/kg ยท IV ยท q8h (for 0.23 mg/kg) or q12h (for 0.35 mg/kg) ยท ARCI UCGFS Class 5 Drug
Routes of administration
Contraindications
- Known hypersensitivity to H2 blockers
- Known hypersensitivity to famotidine or other H2 receptor antagonists
Adverse effects
- Bradycardia (if infused too rapidly IV)
- GI effects (anorexia, vomiting, diarrhea)
- Headache
- Dry mouth or skin
- Intravascular hemolysis (rare, anecdotally reported in cats given IV)
- Transient increase in serum gastrin (returns to baseline by 14 days)
- Generally has a very good safety profile with fewer side effects than cimetidine
Drug interactions
- Azole Antifungals (ketoconazole, itraconazole, fluconazole) ยท Famotidine raises gastric pH, which may decrease the absorption of these agents. Administer the azole one hour prior to famotidine.
- Cefpodoxime, Cefuroxime ยท Famotidine may decrease the absorption of these cephalosporins. Taking with food may alleviate this effect.
- Iron Salts (Oral) ยท Famotidine may decrease the absorption of oral iron. Administer iron at least one hour prior to famotidine.
Monitoring
- Clinical efficacy (decrease in symptomatology, endoscopic examination, resolution of blood in feces)
- Adverse effects
- Clinical signs of GI ulceration or bleeding (vomiting, melena, anorexia)
- Gastric pH (if clinically indicated and feasible)
Overdose
Famotidine has a wide margin of safety. The minimum acute lethal dose in dogs is reported to be >2 grams/kg for oral doses and approximately 300 mg/kg for intravenous doses. IV doses in dogs ranging from 5-200 mg/kg caused vomiting, restlessness, mucous membrane pallor, and redness of the mouth and ears. Higher doses caused hypotension, tachycardia, and collapse. Most overdoses require only monitoring. In massive oral overdoses, gut-emptying protocols should be considered and supportive therapy initiated when warranted.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.