In young adults, phenytoin is usually started at 4-5 milligrams (mg) per kilogram (kg, about 2.2. pounds) of body weight per day (typically 200-300 mg per day, 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.

Patients over age 65 generally require a lower initial dose (usually about 3 mg/kg per day) and particular caution with any increases.

When phenytoin is used alone (that is, as monotherapy), an average long-term dose 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 liquid suspension is absorbed faster than the form in capsules, so once-daily dosing with the liquid is likely to cause too many side effects.

The doctor occasionally may suggest checking the level of phenytoin 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).

If seizures are too frequent to wait for slow increases in the dosage of seizure medicine, a large dose of phenytoin (called a "loading dose") may be given over a period of minutes to hours in a clinic or hospital, and then a normal dose can be continued the next day.