The effect of anticonvulsant drugs which induce liver microsomal enzymes on derived and ingested phenobarbitone levels.

A comparison was made between the levels of derived Phenobarbitone in three groups of patients who were taking Primidone as a single drug, Primidone with Phenytoin and Primidone in combination with Phenytoin and Carbamazepine. The levels of ingested Phenobarbitone when this drug was taken as a single drug were compared with the levels when Phenobarbitone was taken in combination with Phenytoin in two other groups of patients. A significant increase in derived Phenobarbitone levels occurred when Primidone was used in combination with Phenytoin alone or with Phenytoin and Carbamazepine. The highest level occurred in a group of patients taking the three drug combination. There was no significant difference between the levels of ingested Phenobarbitone when this drug was used as single therapy or in combination with Phenytoin. We suggest that the increase in derived Phenobarbitone levels relates to the effect of Phenytoin on liver enzyme systems, and that the greater increase with triple therapy was related to the combined effect of Carbamazepine and Phenytoin on microsomal enzymes. As there was no increase in ingested Phenobarbitone levels when this drug was taken in combination with Phenytoin, we were unable to confirm previous suggestions that Phenytoin either inhibits the hydroxylation of Phenobarbitone or impairs its renal excretion.