Drug interactions: theory versus practice.

Cimetidine inhibition of P-450 oxidative metabolism results in interactions with many drugs, leading to clinically significant drug toxicity. A one-year survey of inpatient prescribing practices revealed a 32.6 percent incidence of concomitant cimetidine and interacting drug use. Retrospective chart review showed statistically significant increased toxic drug plasma levels in patients receiving theophylline and cimetidine, and a strong trend to increased rates of drug toxicity in phenytoin-cimetidine treated patients. Ranitidine appears to be a superior choice in patients receiving drugs metabolized by P-450 oxidation. When cimetidine and known interacting drugs are prescribed together, more frequent assessment of blood levels of the affected drug will be required, increasing substantially the cost to patients. Drug interactions of clinical significance occurring due to altered absorption because of effects of H2-receptor antagonists on gastric secretion are reviewed.