Possible interaction between phenobarbital, carbamazepine and itraconazole

We report a case of a possible interaction between itraconazole, phenobarbital and carbamazepine. The first plasma itraconazole concentration, measured when the patient had been taking phenobarbital for 2 months, was very low. The second measurement, 2 months after withdrawing phenobarbital, was higher but below the therapeutic range. However, carbamazepine, a well known enzyme inducer, had been initiated 15 days before. 20 days after carbamazepine was withdrawn, the itraconazole concentration 4 hours after administration was near the lower end of the therapeutic range. The mechanism of this possible interaction is probably the same for phenobarbital and carbamazepine, involving hepatic microsomal enzyme system induction.