Phenytoin has been used for many years to treat children with partial and generalized tonic-clonic seizures. It is not effective for absence seizures, so children with both tonic-clonic and absence seizures will need combination therapy. Often phenytoin is not chosen as a first-choice medication for children, however, because of troublesome effects on appearance, including gingival hyperplasia and hirsutism. The gum disorder affects almost one-half of children who take phenytoin for a prolonged period and is made worse by braces.

Nystagmus and ataxia are common when blood levels reach the top of the usual therapeutic range. Lethargy, unsteadiness in the feet and hands, dysarthria, vomiting, problems with thinking or behavior, or mood changes are other common dose-related side effects. Because of nonlinear kinetics, small increases in the dose of phenytoin can increase blood levels considerably, exacerbating these side effects.

Dosage increases should be done slowly to minimize side effects. Children usually start taking phenytoin with a dose of 5 mg/kg per day. For most children, this is less than the smallest Phenytek capsule (200 mg). Taking less than a whole capsule of Phenytek is not recommended, so they will probably need to start by taking another form of phenytoin. For many children, the dose is later increased until it totals 200 to 300 mg per day. These children then can use Phenytek instead.