The effects of phenytoin on phenobarbitone and primidone metabolism

Serum concentrations of primidone and its metabolites-phenobarbitone and phenylethylmalonamide-were measured in 40 epileptic patients receiving treatment with primidone alone or primidone plus phenytoin. Serum phenobarbitone concentrations were also measured in 100 patients receiving phenobarbitone, alone or with phenytoin showed raised serum phenobarbitone concentrations. Phenytoin also caused raised phenylethylmalonamide concentrations in patients on primidone.