Strong CYP3A4 inhibitors (ketoconazole, itraconazole, clarithromycin, erythromycin, ritonavir), CYP3A4 inducers (rifampicin, phenytoin, carbamazepine, St John"s wort), other heart rate lowering drugs (beta-blockers, diltiazem, verapamil, digoxin, amiodarone), grapefruit juice