Test Details: Dilantin is a prescription medicine used to treat the symptoms of seizures as a anticonvulsant. Dilantin may be used alone or with other medications. Dilantin belongs to a class of drugs called Anticonvulsants, Hydantoins; Antidysrhythmics, Ib. The phenytoin test is used to optimize drug therapy, monitor the amount of phenytoin in the blood, determine whether drug concentrations are in the therapeutic range, and to monitor patient adherence.