Cimetidine

Provider Level

RN/Paramedic

Class

Histamine H2-receptor antagonist

Mechanism of Action

Competitively inhibits the action of histamine at the H2 receptors of the gastric parietal cells, reducing gastric acid secretion and volume.

Indications

Allergic reactions (anaphylaxis and urticaria) as an adjunct to epinephrine and H1 antagonists. Prevention of aspiration pneumonitis. Gastrointestinal hemorrhage.

Contraindications

Known hypersensitivity to cimetidine or other H2-receptor antagonists.

Routes

PO, IM, IV, IO

Onset / Duration

Onset: 15 to 60 minutes. Peak effect: 1 to 2 hours. Duration: 4 to 5 hours.

Adverse Effects

Headache, dizziness, confusion (especially in elderly patients), bradycardia (with rapid IV administration), hypotension, diarrhea, and cardiac arrhythmias.

Precautions & Interactions

Rapid IV administration may cause bradycardia and hypotension; must administer slowly. Significant hepatic or renal impairment requires extreme caution. Exhibits extensive cytochrome P450 inhibition, leading to numerous severe drug-drug interactions.

Special Considerations

Largely replaced in modern practice by newer H2 antagonists (like famotidine) or proton pump inhibitors due to its extensive side effect and interaction profile. Monitor ECG and vital signs closely during IV administration.