The Phenytek package insert recommends that seizure control be established with divided doses using 100-mg phenytoin capsules before once-daily dosage with Phenytek is considered. In young adults, phenytoin should be started at 4-5 mg/kg per day (200-300 mg/d, in one or two doses) and titrated by increments of 30 mg to 100 mg per day, with at least 7-10 days between changes. Elderly patients generally require a lower initial dose (3 mg/kg per day) and especially cautious titration.

The average maintenance dose as monotherapy for adults is 4-7 mg/kg per day (300-600 mg/d), often given in two doses. The package insert for Dilantin notes that some patients do well on one daily dose of 300 mg, using the capsule form of Dilantin. Phenytek will provide the same dosage in one capsule.

If a new prescription uses capsules with a different dose, remind patients that they should not automatically continue to take the same number of capsules as before. This is not a problem, of course, if the patient is taking only one capsule per day. But patients taking higher doses could make this mistake. If the prescription changed from three 200-mg capsules to two 300-mg capsules, for instance, continuing to take three capsules would be a serious error.

Phenytek capsules (like Dilantin and other phenytoin capsules) are formulated with the sodium salt of phenytoin. Dilantin Infatabs chewable tablets and the Dilantin suspension contain the free acid form of phenytoin, however, which yields about an 8% higher drug content. Switching from tablets or suspension to Phenytek (or vice versa) thus may require dosage adjustments and monitoring.

In children over 6, the usual maintenance dose is 4-8 mg/kg per day, usually in two doses. The form of phenytoin in Dilantin suspension and chewable tablets is absorbed faster than the form in Dilantin capsules, so once-daily dosing with those forms is likely to cause toxicity. Phenytek can be considered for these children if the available doses are appropriate.

A therapeutic blood level of phenytoin is general considered to be 10-20 mcg/mL (lower for seniors), but adjustments should depend on clinical response. At higher levels, small increments in dose may produce large increments in blood level because of saturating kinetics.

• Algorithm for dosing phenytoin (Dilantin, Phenytek)