High doses of
oxcarbazepine increase
phenytoin levels, and a reduction in the
phenytoin dose may be required. Elevations in
phenytoin levels of up to 40% have been seen in retrospective studies. Fosphenytoin, a prodrug of
phenytoin, may interact similarly.
Bear this interaction in mind on concurrent use, and with high
oxcarbazepine doses consider monitoring
phenytoin levels. Indicators of
phenytoin toxicity include blurred vision, nystagmus, ataxia or drowsiness.