Phenytoin increases the clearance of
topiramate 2- to 3-fold, which appears to result in
topiramate levels that are up to 50% lower.
Topiramate slightly increases the
phenytoin AUC (raised about 25%) but this is said not to be clinically significant. Fosphenytoin, a prodrug of
phenytoin, may interact similarly.
Topiramate dose adjustments may be required if
phenytoin is added or discon- tinued. Be aware that a few patients may have increased
phenytoin levels, particularly at high
topiramate doses. Warn the patient to monitor for indicators of
phenytoin toxicity (blurred vision, nystagmus, ataxia or drowsiness). Consider monitoring
phenytoin levels.