Cimetidine
Also known as: Zitac · Tagamet · Cimetidinum
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Audited v13 dosing evidence
Structured calculator evidenceGI ulceration (extra-label)
- q8-12h
NA
Governing clinical context (1)
- Cimetidine products should be stored protected from light in airtight containers and kept at room temperature, 20°C to 25°C (68°F-77°F).
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
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 of action
Acts as a Histamine (H2) receptor antagonist → competitively inhibits histamine at H2 receptors of the gastric parietal cells → 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.
Safety & warnings
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)
Precautions
Warning: Rebound gastric acid secretion may be seen on cessation of cimetidine, so therapy should be tapered. Concomitant treatment with sucralfate may be helpful.
Warning: If used IV, should be administered slowly over 30 minutes to prevent cardiac arrhythmias and hypotension.
- Dosage should be reduced for animals with renal impairment.
Drug interactions
Increases plasma levels of beta-blockers due to CYP450 inhibition
Increases plasma levels of calcium-channel blockers
Increases plasma levels of diazepam
Increases plasma levels of lidocaine
Increases plasma levels of metronidazole
Increases plasma levels of pethidine
Increases plasma levels of theophylline
May exacerbate leucopenia
May decrease cimetidine bioavailability; administer sucralfate at least 2 hours before
May decrease absorption; stagger oral doses by 2 hours
Altered absorption/metabolism; stagger oral doses by 2 hours
Decreased absorption of itraconazole due to reduced gastric acidity; stagger oral doses by 2 hours
Potential interaction; stagger oral doses by 2 hours
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)
Pharmacokinetics
Half-life
Absorption
Rapidly absorbed with high bioavailability.
Distribution
Widely distributed.
Metabolism
Undergoes hepatic metabolism. Retards oxidative hepatic drug metabolism by binding to the microsomal cytochrome P450.
Elimination
Renal excretion.
Overdose
Adverse reactions are generally minor even at high doses. Supportive care should be provided if massive overdose occurs.
Available products
Formulations
- Injectable: 100 mg/ml solution in 2 ml ampoule
- Oral: 100 mg tablets
- Oral: 200 mg tablets
- Oral: 40 mg/ml syrup
Veterinary
- Zitac
Human-labeled
- Tagamet
- Cimetidine generics
Regulatory status
Available as human generic OTC and Rx.
POM-V, POM
POM-V, POM
No regulatory data for: 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Normal precautions should be observed. Store at room temperature away from light and moisture.
Client information
Cimetidine is used to treat stomach ulcers, acid reflux, and conditions where the stomach produces too much acid.
- Do not stop this medication abruptly, as it can cause a rebound in stomach acid production. Your vet will likely instruct you to taper the dose gradually when stopping treatment.
- If you are also giving sucralfate, antacids, or certain other medications, separate the doses by at least 2 hours to ensure both drugs are absorbed properly.
- Side effects are rare, but contact your veterinarian if you notice any unusual signs, such as unexplained bruising (which could indicate low platelets) or swelling of the mammary glands in female dogs.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.