Cimetidine for Dogs – H2 Blocker
Cimetidine is an older H2 receptor antagonist largely replaced by famotidine (longer-acting, fewer interactions) and omeprazole (PPI – more effective).
Dose
5-10 mg/kg PO every 6-8 hours
Dose Reference Table
| Weight | Range |
|---|---|
| 5 kg | 25-50 mg |
| 10 kg | 50-100 mg |
| 20 kg | 100-200 mg |
| 30 kg | 150-300 mg |
| 40 kg | 200-400 mg |
Tablets: 200, 300, 400, 800 mg. Tagamet HB 200 mg OTC.
Indications
- GI ulcer prophylaxis
- Gastritis / esophagitis
- Acid reflux
- Acetaminophen toxicity (CYP450 inhibition)
- Mast cell tumor adjunct (histamine effects)
- Stress ulcer prevention
Cimetidine vs Famotidine
| Feature | Cimetidine | Famotidine |
|---|---|---|
| Dosing | Q6-8H | Q12-24H |
| Drug interactions | Many (potent CYP450 inhibitor) | Few |
| Acid suppression | Less | Better |
| Cost | Similar | Similar |
| Preferred? | When specific indication | Yes for most |
Acetaminophen Toxicity Protocol
- Cimetidine 5-10 mg/kg q6h x 7 days (CYP450 inhibition)
- NAC 140 mg/kg loading then 70 mg/kg q6h (primary antidote)
- SAMe (Denamarin) – hepatic protection
- IV fluids support
- Supportive care
- Particularly important in CATS (very susceptible)
âš Drug Interactions (CYP450 Inhibitor)
Check before combining:
- Warfarin – increased anticoagulation
- Propranolol/metoprolol – increased
- Diazepam/midazolam – increased sedation
- Phenytoin – increased
- Theophylline – toxicity risk
- Phenobarbital – increased
- Morphine – increased
- Many others
Side Effects
Common (mild)
- Diarrhea
- Mild GI upset
Uncommon
- Sedation
- Bradycardia (high IV dose)
- Hepatic enzyme elevations
Long-term human use (low dog risk)
- Gynecomastia
- Decreased libido
Frequently Asked Questions
How much cimetidine for my dog?
DOSE: 5-10 mg/kg PO every 6-8 hours. EXAMPLES: 1) 5 kg = 25-50 mg per dose; 2) 10 kg = 50-100 mg; 3) 20 kg = 100-200 mg; 4) 30 kg = 150-300 mg; 5) 40 kg = 200-400 mg. TABLETS: 200, 300, 400, 800 mg (Tagamet HB 200 mg OTC). With or without food. Q6-8H dosing inconvenient – FAMOTIDINE often preferred (Q12-24H, fewer drug interactions). For ACETAMINOPHEN TOXICITY: 5-10 mg/kg q6h x 7 days as adjunct to NAC + SAMe + supportive care. Check all medications for interactions before starting (potent CYP450 inhibitor).
Cimetidine vs famotidine – which is better for my dog?
FAMOTIDINE generally PREFERRED. ADVANTAGES of famotidine over cimetidine: 1) ONCE-TWICE daily dosing (vs Q6-8H cimetidine) – better compliance; 2) FEWER drug interactions (cimetidine potent CYP450 inhibitor); 3) BETTER acid suppression; 4) Less GI motility effect; 5) Better safety profile; 6) Similar generic cost. WHEN TO USE CIMETIDINE: 1) ACETAMINOPHEN TOXICITY – CYP450 inhibition specifically reduces NAPQI toxic metabolite formation (unique benefit); 2) MAST CELL TUMOR with significant histamine release; 3) Famotidine unavailable; 4) Specific protocols. WHEN TO USE FAMOTIDINE: 1) Routine acid suppression; 2) GI ulcer prophylaxis; 3) Gastritis; 4) Most situations. WHEN TO USE OMEPRAZOLE (PPI): 1) Severe ulcers; 2) Bleeding ulcers; 3) Reflux refractory to H2 blockers; 4) Mast cell tumor severe; 5) NSAID-induced ulcers; 6) Most effective acid suppression. Discuss with vet for specific case.
Can cimetidine treat acetaminophen poisoning in my dog?
YES – as adjunct to NAC + supportive care. ROLE: 1) Inhibits CYP450 hepatic enzymes; 2) Reduces formation of NAPQI (toxic acetaminophen metabolite); 3) Decreases hepatic damage; 4) Adjunctive only – NOT primary treatment. ACETAMINOPHEN TOXICITY PROTOCOL: 1) DECONTAMINATION if recent (emesis if conscious within 1-2 hours, activated charcoal); 2) NAC (N-acetylcysteine) – PRIMARY antidote: 140 mg/kg PO loading then 70 mg/kg q6h x 7 doses; 3) CIMETIDINE 5-10 mg/kg PO q6h x 7 days (CYP450 inhibition); 4) SAMe (Denamarin) – replenishes glutathione; 5) ASCORBIC ACID (vitamin C) 30 mg/kg q6h – antioxidant; 6) IV fluids; 7) Monitor liver function + methemoglobinemia + Heinz body anemia; 8) Hospitalization for supportive care. CATS vs DOGS: 1) CATS extremely susceptible (lack glucuronidation); 2) Dogs less susceptible but still toxic at high doses; 3) Earlier treatment = better outcome; 4) Time-critical protocol. SIGNS of acetaminophen toxicity: 1) Methemoglobinemia (chocolate-brown mucous membranes, blue gums); 2) Heinz body anemia (RBC damage); 3) Hepatic failure; 4) Facial/paw edema; 5) Vomiting; 6) Hypoxia, weakness. EMERGENCY at any suspected ingestion – immediate vet visit + decontamination + protocol initiation. Don’t delay treatment – hepatic damage progresses.
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References & Further Reading
The dosing ranges and safety information on this page are drawn from the following veterinary references. Always defer to your own veterinarian and the manufacturer’s label for your specific product.
- Plumb DC. Cimetidine.
- Schmiedt CW et al. H2 blockers in veterinary medicine.
- Hsu CC. Acetaminophen toxicity management.
- Marks SL et al. Acid suppression.
- PuppaDogs. Famotidine Calculator, Acetaminophen Toxicity Calculator. puppadogs.com.
