An antiepileptic drug, zonisamide, which has a wide antiepileptic spectrum, is commonly prescribed concomitantly with other antiepileptic drugs. However, there appear to be a little data on the drug interaction between zonisamide and other antiepileptic drugs. Especially, the effect of zonisamide on the plasma concentration of phenobarbital in epileptic patients has not yet been reported. The effect of zonisamide on the plasma concentration of phenobarbital was therefore investigated in epileptic patients. The ratio of plasma phenobarbital concentration to the administered dose of phenobarbital when coadministered with zonisamide was found to be significantly higher than that of phenobarbital alone (P<0.05). The plasma concentration of phenobarbital was increased by zonisamide in a dose-dependent fashion. The present findings provide useful information that the monitoring of the plasma concentration of phenobarbital is needed to adjust the dosage regiments of phenobarbital in combination with zonisamide.
