Increased plasma conc w/ CYP3A4 inhibitors (eg, itraconazole, ketoconazole, voriconazole, posaconazole, HIV protease inhibitors, clarithromycin, telithromycin, nefazodone), P-gp inhibitors (eg, ciclosporin, quinidine, verapamil), & CYP2D6 inhibitors. Increased plasma digoxin conc by an average of 1.5-fold. Possible risk of ventricular arrhythmias increased w/ drugs known to prolong the QTc interval eg, antiarrhythmics (eg, quinidine, disopyramide, procainamide), erythromycin & TCAs (eg, imipramine, doxepin, amitriptyline).