Phenytoin is highly effective for seizures and is one of the most commonly used seizure medicines in North America. It is best used for partial-onset seizures, which begin in one area of the brain. Its effectiveness in controlling seizures has been extensively studied in careful scientific trials in great numbers of patients.

Many studies have compared phenytoin with other seizure medicines, to see which medicine is best for people who have just begun treatment for epilepsy. On average, the results were about the same for phenytoin as for several other seizure medicines that are often used.

Differences in side effects may be important in deciding which medicine is best for each person. Because some people do better with phenytoin and others do better with something else, it's difficult to forecast the results in any given person.

If seizures continue, or if bothersome side effects occur, the doctor probably will change the amount of phenytoin prescribed. If that doesn't work, the next step may be either to prescribe a different seizure medicine by itself or to prescribe a combination of phenytoin and another seizure medicine. Many are available. No single combination is best for everyone.