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). The package insert for brand-name Dilantin indicates that this amount is usually given in three or four doses, though it notes that some patients do well on one daily dose of 300 mg, using the capsule form. Once-daily dosing also can be achieved using Phenytek, an extended-release form of phenytoin.

In children over 6, the usual maintenance dose is 4–8 mg/kg per day, usually in two doses. The form of phenytoin in the suspension is absorbed faster than the form in phenytoin capsules, so once-daily dosing with the suspension is likely to cause toxicity.

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.

If therapeutic levels need to be reached quickly, a loading dose can be administered orally or parenterally. An oral loading dose consists of four doses (400 mg, 300 mg, 300 mg) given at 2-hour intervals in a clinic or hospital, where serum levels can be closely monitored. A normal maintenance dose is begun 24 hours later.

• Algorithm for dosing phenytoin (Dilantin, Phenytek)