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 with a dose of 5 mg/kg per day. This is usually given in two or three equally divided doses. Most children do best at about 4 to 8 mg/kg per day. The highest dose recommended for young children is 300 mg per day, which can be given using the suspension. Children older than 6 years and adolescents may require the minimum adult dosage of 300 mg per day.

If the suspension is used, the importance of thoroughly shaking the bottle immediately before measuring must be stressed, since phenytoin is poorly soluble and dosing errors can occur.

The use of oral phenytoin is not recommended for infants because of problems with bioavailability and a narrow therapeutic index.