Phenytoin has been used for many years to treat children with partial and generalized tonic-clonic seizures. It can cause side effects from high doses and allergic reactions at any dosage. Often it is not chosen as a first-choice medication for young children because it may have troublesome effects on appearance when used for a long time.

The first sign of an allergic reaction to phenytoin is a rash, so if you see any skin problems, call a doctor immediately. In rare cases, allergic reactions to phenytoin can result in damage to the liver or bone marrow.

The most common side effect related to phenytoin in children is jerky movements of the eyes called nystagmus. These movements often do not interfere with a child's vision.

Other side effects from high doses include unsteadiness in the feet and hands, sleepiness, and vomiting. These may be avoidable if the dosage is increased slowly. They should quickly disappear when the dosage is reduced.

In a few children, phenytoin causes problems with thinking or behavior, mood change, slow or clumsy movements, or a loss of energy.

One fairly common side effect in children is overgrowth of the gums. This occurs in almost one-half of children. Braces can make this problem even more severe. Thorough brushing, daily flossing, and regular visits to the dental hygienist will diminish the swelling. A few children need gum surgery.

Some young people who take fairly high doses of phenytoin also have increased growth and darkening of hair on the face and body, called hirsutism. These effects are especially troublesome for young women.

The doctor may prescribe the liquid form of phenytoin for young children who have trouble swallowing pills. Children usually start with a dose of 5 milligrams (mg) per kilogram (kg, about 2.2 pounds) of their body weight per day. This is usually given in two or three equally divided doses.

Most children do best at about 4 to 8 mg per kg per day. The highest dose recommended is 300 mg per day. Children older than 6 years and adolescents may require the minimum adult dosage of 300 mg per day.