Cimetidine
Cimetidine
Brand names:Tagamet, Tagamet HB
A first-generation histamine H2 receptor antagonist that reduces gastric acid secretion. Cimetidine was the original H2 blocker but has largely been displaced by famotidine in modern practice due to its extensive CYP450 drug interaction profile and shorter duration. It remains on some tactical EMS supplemental formularies for acid suppression and as an adjunctive agent in anaphylaxis management.
Pharmacology and Actions
Cimetidine competitively antagonizes histamine at H2 receptors on gastric parietal cells, reducing gastric acid secretion in response to histamine, gastrin, and acetylcholine stimulation. In anaphylaxis management, H2 blockade is adjunctive to epinephrine and H1 blockade and provides additional symptomatic relief for cutaneous and GI manifestations. Cimetidine also inhibits multiple CYP450 isoenzymes (CYP1A2, CYP2C9, CYP2D6, CYP3A4), which produces its extensive drug interaction profile.
Indications
- GERD and acid-related dyspepsia
- Peptic ulcer disease (active or maintenance)
- Adjunctive treatment of anaphylaxis (H2 component, paired with H1 antihistamine after epinephrine)
- Stress ulcer prophylaxis in critically ill patients
Absolute Contraindications
- Known hypersensitivity to cimetidine
Precautions and Side Effects
Common: headache, dizziness, diarrhea, rash. CNS: confusion and delirium in elderly or renally impaired patients (cimetidine crosses the blood-brain barrier more readily than famotidine). Endocrine: gynecomastia and impotence with chronic high-dose use due to androgen receptor effects. Hematologic (rare): thrombocytopenia and agranulocytosis. The major operational concern is the CYP450 inhibition profile, which increases levels of warfarin, theophylline, phenytoin, lidocaine, propranolol, metronidazole, tricyclic antidepressants, benzodiazepines, and many other commonly co-administered drugs. Reduces absorption of pH-dependent drugs (ketoconazole, atazanavir, iron, B12). Half-life 2 hours, prolonged in renal impairment - reduce dose by 50 percent if CrCl under 30. Pregnancy Category B. Compatible with lactation, though famotidine is preferred. Monitor mental status in elderly, renal function, and CBC with prolonged use. For most modern indications, famotidine is preferred over cimetidine due to fewer interactions and longer duration.
Adult Dosing
Pediatric Dosing
20 to 40 mg/kg/day PO divided every 6 hours. Not commonly used in pediatric tactical EMS practice.
Pharmacokinetics
Peak Effect: PO: 1 to 1.5 hours. IV: 15 to 60 minutes.
Duration: 4 to 5 hours - shorter than famotidine, which contributed to its replacement in most modern protocols.
Storage and Handling
Store at room temperature. Protect from light and moisture. Injectable formulation should not be frozen.
Reconstitution:
IV solution can be given undiluted slowly (over at least 5 minutes) or diluted in 20 mL of compatible IV fluid. Rapid IV push has been associated with hypotension and arrhythmia.
TCCC and TECC Role
Cimetidine is not in the TCCC core formulary. It appears in tactical EMS supplemental formularies as an H2 component for adjunctive anaphylaxis management and for operational acid suppression. The H2 role in anaphylaxis is adjunctive only - epinephrine remains first-line and life-saving; H2 blockade adds modest benefit to H1 blockade for cutaneous and GI symptoms after epinephrine is on board. For TECC settings, famotidine is generally preferred when available.
Cimetidine sits in a complicated place in modern tactical medicine. It was the first H2 blocker and dominated for decades, but famotidine displaced it in most clinical practice due to fewer drug interactions and longer dosing interval. Some TEMS formularies still list cimetidine, often as a legacy choice. For operational acid suppression, omeprazole or famotidine are usually preferred. The drug interaction profile means cimetidine in an aid bag is essentially a liability with operators on any chronic medication. Reach for famotidine or omeprazole instead when available.
Treating cimetidine as a low-risk OTC heartburn medication and overlooking the CYP450 interactions. An operator on warfarin, phenytoin, theophylline, or a tricyclic antidepressant can get into serious trouble with a few doses of cimetidine. The other mistake is using it as monotherapy for anaphylaxis - it is adjunctive only, and giving it instead of (or before) epinephrine is dangerous.
This drug profile is provided as educational reference material for trained medical providers. It is not medical advice, not a substitute for formal training, and not a substitute for current published guidelines or medical direction.
Drug administration is governed by your scope of practice, agency standing orders, medical director protocols, and applicable state and federal regulations. Controlled substances are subject to additional handling, accountability, and documentation requirements per DEA and state law. Always verify dosing, indications, contraindications, and route of administration against current published guidelines and your local protocols before administration.
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Drug information evolves. Last reviewed dates and source citations are provided for each entry. Confirm currency against the cited source before clinical use.
Penn Tactical Solutions publishes this reference for educational purposes. PTS does not provide medical direction and does not assume responsibility for clinical decisions made in the field. Clinical responsibility rests with the administering provider, their medical director, and their agency.
Educational reference for trained medical providers. Not medical advice. Not a substitute for formal training, current published guidelines, or medical direction. Drug administration is governed by your scope of practice, agency standing orders, medical director protocols, and applicable state and federal regulations. Controlled substances require additional storage, accountability, and documentation per DEA and state law.
In a medical emergency, call 911. This reference is not a substitute for emergency medical services.
Verify dosing, indications, and contraindications against current published guidelines and your local protocols before administration. Confirm content currency against the source citation. Penn Tactical Solutions does not provide medical direction. Clinical responsibility rests with the administering provider, their medical director, and their agency.
Cimetidine
| IV/IO | Acid suppression in hospitalized patient: 300 mg IV every 6 to 8 hours, administered slowly over at least 5 minutes (rapid push has been associated with hypotension and arrhythmia). Anaphylaxis adjunct: 300 mg IV over 5 minutes, paired with H1 antihistamine such as diphenhydramine 25 to 50 mg IV after epinephrine has been administered. (15 minutes) |
| IM | 300 mg IM every 6 to 8 hours, though IV is preferred when access is available. (15 to 30 minutes) |
| IN | None (None) |
| PO | OTC: 200 mg PO once or twice daily for heartburn. Prescription: 300 mg PO four times daily, or 400 mg twice daily, or 800 mg at bedtime for active ulcer. Maintenance: 400 mg PO at bedtime. (30 to 60 minutes) |