Influenza
vaccine is reported to increase, decrease or to have no effect on the
serum levels of
phenytoin. Fosphenytoin, a prodrug of
phenytoin, may interact similarly. The
efficacy of the
vaccine is unaffected.
Phenytoin levels appear to return to normal after about 14 days. Warn the patient to monitor for indicators of
phenytoin toxicity (blurred vision, nystagmus, ataxia or drowsiness). However, be aware that any alteration in levels may take a couple of weeks to develop and usually resolves spontaneously.