Isoniazid as an inhibitor of primidone metabolism.

Isoniazid inhibited the metabolism of primidone in a patient with focal seizures. The steady-state serum level of primidone rose when the patient received both drugs simultaneoulsy. The serum levels of the primidone metabolites, phenobarbital and phenylethylmalonamide, fell and the rate of metabolism of primidone decreased. The results are similar to those observed when isoniazid is adminstered with diphenylhydantoin.