Various forms of phenytoin, the medicine in Phenytek, have been used for many years to treat children with partial seizures and generalized tonic-clonic seizures. It is not effective for absence seizures, so children with both tonic-clonic and absence seizures will need to take another seizure medicine as well (combination therapy). Often Phenytek is not chosen as a first-choice seizure medicine for children, however, because of its troublesome effects on appearance (see common side effects). Almost one-half of children who take Phenytek and other forms of phenytoin for a prolonged period experience overgrowth of the gums, which is made worse by braces.

Jerking of the eyes and problems with coordination are common when the amount of Phenytek in the blood nears the top limit usually considered effective. Because of the way the body digests this medicine, at certain levels a small increase in the dose given can raise the amount in the blood a great deal. This can make side effects much worse.

To keep side effects at a minimum, children usually start taking phenytoin with a dose of 5 milligrams (mg) per kilogram (kg, equal to 2.2 pounds) of their body weight per day. For most children, this is less than the smallest Phenytek capsule (200 mg), so they will probably start by taking two or three equal doses of another form of phenytoin, such as Dilantin. For many children, the dose is later increased until it totals 200 to 300 mg per day. These children then can use Phenytek instead. Since it is only available in capsules that should be swallowed whole, however, it is seldom prescribed for small children.

After they've been taking phenytoin (Dilantin etc.) for a while, most children do best at about 4 to 8 mg per kg per day. Many older children and teenagers may require the minimum adult dosage of 300 mg per day, so they can use Phenytek.