The QT interval prolonging effects are increased when procainamide
are used together (see drugs that prolong the QT interval, for a general discussion of QT prolongation). Amiodarone
increases the levels of procainamide
and its metabolite
by 60% and 30%, respectively.
Concurrent use should generally be avoided due to the QT prolonging effects of the combination. If the two drugs are considered essential, the dosage of procainamide
may need to be reduced by 20 to 50%. Levels should be monitored where possible, and patients observed closely for adverse effects.