Quinidine levels can be reduced by
phenytoin. Loss of arrhythmia
control is possible if the
quinidine dosage is not increased. Fosphenytoin, a prodrug of
phenytoin, may interact similarly.
Concurrent use need not be avoided, but be alert for the need to increase the
quinidine dosage. If
phenytoin is withdrawn the
quinidine dosage may need to be reduced to avoid
quinidine toxicity.
Quinidine serum levels should be monitored if possible.