Phenytoin pharmacokinetics

Nonlinear; elimination kinetics shift from first-order to zeroorder at moderate to high dose levels. The drug binds extensively to plasma The drug is 90 percent bound to plasma albumin.The oral bioavailability of phenytoin is variable because of individual differences in first-pass metabolism. Rapid-onset and extended-release forms are available. Phenytoin metabolism is proteins (97–98%), and free (unbound) phenytoin levels in plasma are increased transiently by drugs that compete for binding (eg, carbamazepine, sulfonamides, valproic acid). The metabolism of phenytoin is enhanced in the presence of Phenytoin induces drugs metabolized by the CYP2C and CYP3A families and the UGT enzyme system. Phenytoin itself induces hepatic drug metabolism, decreasing the effects of other antiepileptic drugs including carbamazepine, clonazepam, and lamotrigine.