Ca antagonists (verapamil & diltiazem), antiarrhythmics eg, disopyramide. Increased risk of bradycardia w/ digitalis glycosides. Catecholamine-depleting drugs eg, reserpine, guanethidine; clonidine. CYP2D6 inhibitors (eg, quinidine, propafenone, fluoxetine, paroxetine). Increased plasma levels w/ cimetidine. Decreased AUC & C
max of sildenafil.