Diazepam treatment does not influence the debrisoquine or mephenytoin hydroxylation phenotyping tests

Debrisoquine and S-mephenytoin hydroxylation phenotyping tests were performed in 9 patients before and during treatment with diazepam at a daily dose of 10 or 15 mg. There was no change in either the debrisoquine/4-hydroxydebrisoquine ratio or the ratio of S/R mephenytoin in urine. The hydroxylation of these two test drugs is apparently not inhibited by administration of therapeutic doses of diazepam.