- QT Prolongation: Macrilen™ causes an increase of about 11 msec in the corrected QT (QTc) interval. QT prolongation can lead to development of torsade de pointes-type ventricular tachycardia with the risk increasing as the degree of prolongation increases. The concomitant use of Macrilen™ with drugs that are known to prolong the QT interval should be avoided