Because cimetidine is a potent inhibitor of several cytochrome P450 enzymes, contraindication screening must extend beyond allergy history to a full medication reconciliation.
Absolute contraindications include hypersensitivity to cimetidine and concurrent use of medications with narrow therapeutic windows that depend on CYP1A2, 2C19, or 3A4 metabolism when dose adjustments are not feasible. Relative precautions involve renal impairment, existing QT prolongation, and neurologic vulnerability.
Rare but includes urticaria, angioedema; switch to alternative H2 blocker.
Reduce dose when CrCl <50 mL/min/1.73 m² to prevent CNS effects (confusion, agitation).
Cimetidine inhibits multiple cytochrome P450 enzymes; monitor for toxicity with phenytoin, theophylline, warfarin, or tricyclic antidepressants.
Use cautiously due to potential elevation of transaminases; monitor LFTs in prolonged courses.