Dilantin®, Phenytek® (Phenytoin)
One of the oldest and still most widely used anti-epileptic drug (AED) used in America is Dilantin®, Phenytek® (Phenytoin). Phenytoin works for epilepsy by blocking voltage-sensitive sodium (Na+) channels. It is usually thought of as a partial onset seizure medication although it does have activity in some generalized seizure disorders.
Phenytoin is available as a long acting capsule, chewable tablet, and liquid suspension. Phenytek® capsule sizes now include 200 mg and 300 mg in addition to the original Dilantin® capsule sizes of 100 mg and 30 mg. It is also available as an intravenous preparation and it is this ability to give the patient an initial dose intravenously and then convert them to the oral form that has helped it maintain its popularity despite a host of long term side effect problems.
The intravenous form was associated with some significant toxicity unique to its method of administration and more recently a safer pro-drug form, Cerebyx® (Fosphenytoin), has become available for use. Phenytoin causes birth defects and all women of childbearing age should be taking extra folic acid to help minimize the risk of neural tube defects. The medication also lowers the effectiveness of birth control pills by increasing the metabolism of the estrogen component and therefore it is recommended not to use the lowest dose estrogen compounds available. Women who particularly do not want to get pregnant should consider using a barrier method in addition to their birth control pill. The patient discussing her plans for pregnancy before trying to become pregnant minimizes risk to the fetus. Phenytoin has the potential to cause liver toxicity as well as some serious forms of skin rash. More commonly, however, it causes gum growth, increased facial hair, and bone mineral loss. Needless to say it is not the ideal cosmetic medication. When the dose gets too high, the most common complaint is walking like one is drunk or being dizzy.
Phenytoin has been used in the past to treat neuropathic pain but its use has decreased since the introduction of so many other AEDs that have more pain relief potential.