Considerations before Prescribing Phenytoin

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Chaichan Sangdee


Although phenytoin has been used to treat epilepsies for several dacades, it is still among the drug of choice in the treatment of generalized tonic-clonic, simple partial and complex partial seizures, since it is highly efficacious and not expensive. However, phenytoin is a drug with complex pharmacokinetic properties such as its absorption and metabolism may be saturated at the conventional doses and at the upper therapeutic concentrations and high plasma protein binding (approximately 90%). These characteristics can cause serious drug interactions and has great impact on plasma phenytoin concentration. In addition, preparations of phenytoin from different manufacturers may differ significantly in bioavailability and hence are not bioequivalent and thus cannot be used interchangably. All these can lead to problems in clinical setting if physicians and pharmacists are not aware of these special pharmacokinetic characteristics of phenytoin. There are immediate release oral phenytoin preparations such as Ditoin® and modified release preparations such as Dilantin Kapseals®. Immediate release phenytoin preparations have several advantages over those modified release preparations, they are absorbed more rapidly and the extents of absorption are greater and less fluctuation of plasma levels of phenytoin. However, immediate release phenytoin preparations need more frequent dosing, 2-3 times a day while the modified release preparation can be giving once daily. Immediate release preparations may be more suitable when loading dose is deem necessary for the rapid control of frequent seizures.


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