Three cases have occurred in which
ethosuximide appeared to have been responsible for increasing
phenytoin levels, leading to the development of
phenytoin toxicity in 2 patients.
Phenytoin may reduce the
half-life of
ethosuximide (by 50% in one study). Fosphenytoin, a prodrug of
phenytoin, may interact similarly.
Warn the patient to monitor for indicators of
phenytoin toxicity (blurred vision, nystagmus, ataxia or drowsiness). Take
phenytoin levels as necessary, and monitor the effectiveness of
ethosuximide treatment.