Influence of phenobarbital treatment on cimetidine kinetics

The pharmacokinetics of orally administered cimetidine was studied in 8 healthy subjects before and after 3 weeks of treatment with phenobarbital 100 mg daily, and in a separate study 4 subjects received cimetidine intravenously before and after the administration of phenobarbital. There was no change in the volume of distribution, but total plasma clearance was increased by a mean of 18%, mainly due to a 37% increase of nonrenal clearance. Renal clearance and half-life were not significantly altered. The area under the plasma concentration-time curve after oral administration was significantly (P less than 0.05) reduced by a mean of 15% after phenobarbital treatment. The amount of cimetidine excreted in urine and its sulphoxide metabolite were significantly (P less than 0.05) reduced, on average by 34% and 26%, respectively by phenobarbital treatment. The data indicate that an apparent 20% reduction in the absorption of cimetidine was due to induction of gastrointestinal metabolism of cimetidine, with some contribution also from hepatic metabolism. Reduced absorption per se could not be totally excluded. Although the magnitude of the change was small, the finding of an 11% decrease in the time to achieve an effective plasma level of cimetidine after phenobarbital treatment may contribute to the ineffectiveness of cimetidine in certain patients.