Cimetidine
**Cimetidine** is a histamine (H2) receptor antagonist used for the management of idiopathic, uraemic, or drug-related erosive gastritis, gastric and duodenal ulcers, oesophagitis, and hypersecretory conditions secondary to gastrinoma, mast cell neoplasia, or short bowel syndrome. > **Clinical Warning:** Efficacy against NSAID-induced ulcers is controversial. It is less effective at reducing gastric acidity than more modern H2 blockers (e.g., famotidine) and proton pump inhibitors (e.g., omeprazole). It has minimal prokinetic effects and is not an antiemetic.
Mechanism: Acts as a **Histamine (H2) receptor antagonist** โ competitively inhibits histamine at H2 receptors of the gastric parietal cells โ reduces histamine-induced gastric acid secretion. It also binds to microsomal **cytochrome P450** enzymes, retarding oxidative hepatic drug metabolism. It has weak anti-androgenic effects.
Dosing by species
- All uses (gastritis, ulcers, oesophagitis, hypersecretory conditions) ยท 2.5-5 mg/kg ยท PO/IV/IM ยท q12h ยท Taper upon cessation ยท Administer IV over 30 minutes. Reduce dose in renal impairment.
- All uses (gastritis, ulcers, oesophagitis, hypersecretory conditions) ยท 5 mg/kg ยท PO/IV/IM ยท q8h ยท Taper upon cessation ยท Administer IV over 30 minutes. Reduce dose in renal impairment.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Routes of administration
Contraindications
- No specific contraindications available, but use with caution in patients with known hypersensitivity.
Adverse effects
- Thrombocytopenia (reported in dogs)
- Transient and self-resolving slight swelling of mammary glands (female dogs)
- Hepatotoxicity (rare, reported in humans)
- Nephrotoxicity (rare, reported in humans)
- Headache (humans)
- Decreased libido (humans)
Drug interactions
- Propranolol ยท Increases plasma levels of beta-blockers due to CYP450 inhibition ยท moderate
- Verapamil ยท Increases plasma levels of calcium-channel blockers ยท moderate
- Diazepam ยท Increases plasma levels of diazepam ยท moderate
- Lidocaine ยท Increases plasma levels of lidocaine ยท major
- Metronidazole ยท Increases plasma levels of metronidazole ยท moderate
- Pethidine ยท Increases plasma levels of pethidine ยท moderate
- Theophylline ยท Increases plasma levels of theophylline ยท major
- Myelosuppressive agents ยท May exacerbate leucopenia ยท moderate
- Sucralfate ยท May decrease cimetidine bioavailability; administer sucralfate at least 2 hours before ยท minor
- Antacids ยท May decrease absorption; stagger oral doses by 2 hours ยท minor
- Digoxin ยท Altered absorption/metabolism; stagger oral doses by 2 hours ยท moderate
- Itraconazole ยท Decreased absorption of itraconazole due to reduced gastric acidity; stagger oral doses by 2 hours ยท moderate
- Maropitant ยท Potential interaction; stagger oral doses by 2 hours ยท minor
Monitoring
- Clinical signs of GI ulceration or bleeding
- Renal function (to guide dose reduction if impaired)
- Heart rate and blood pressure (if administering IV)
- Complete blood count (monitor for thrombocytopenia or leucopenia, especially if on concurrent myelosuppressive drugs)
Overdose
Adverse reactions are generally minor even at high doses. Supportive care should be provided if massive overdose occurs.
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.