Some people with newly diagnosed epilepsy may start taking Phenytek immediately, but often a different form of the same medicine, phenytoin, is prescribed first. In young adults, phenytoin is usually started at 4-5 milligrams (mg) per kilogram (kg, about 2.2. pounds) of body weight per day. This typically works out to 200 to 300 mg per day, given in one or two doses. After 7 to 10 days, the amount can be increased by 30 to 100 mg per day as required, if side effects are not troublesome.

When seizure control is established, the doctor may switch the prescription from the other form of phenytoin (such as Dilantin) to Phenytek. For instance, a person who has been taking three 100-mg Dilantin Kapseals usually can switch to one 300-mg capsule of Phenytek. The doctor may want to check the level of medication in the blood during the change, however, because some adjustment of the dosage may be needed.

People over age 65 generally need to start with a lower dose (usually about 3 mg/kg per day) and be especially cautious with any increases. Because this kind of medication stays in the body longer, seniors often do well taking Dilantin or other phenytoin capsules once a day. They can also use Phenytek, but its advantages may be less for them than for younger people.

In children over 6, the usual long-term dose of phenytoin is 4-8 mg/kg per day. A daily 200-mg or 300-mg capsule of Phenytek may be appropriate, depending on the child's size and other needs. Because children's bodies digest the medication quickly, children who take Dilantin or other types of phenytoin generally need to take them at least twice a day.

The doctor occasionally may suggest checking the level of Phenytek in the blood to see whether it falls within the range that is usually considered appropriate for treatment ("therapeutic level"). Whether the amount prescribed needs to be adjusted generally depends on whether seizures or side effects are occurring, however. When the level in the blood is fairly high, a small increase in the dose may produce a large increase in the level in the blood (often causing side effects).