Serum
disopyramide levels are reduced by
phenytoin and may fall below therapeutic levels. Loss of arrhythmic
control may occur. Fosphenytoin, a prodrug of
phenytoin, may interact similarly.
Antiarrhythmic response and
serum disopyramide levels (where possible) should be well monitored. Adjust the
disopyramide dose as necessary.
Serum disopyra- mide levels appear to return to normal within 2 weeks of withdrawing the
phenytoin.