CONCOMITANT ADMINISTRATION OF OTHER B-2 AGONISTS WITH CORTICOSTEROIDS, DIURETICS OR XANTHINES INCREASE THE RISK OF HYPOKALEMIA.TRICYCLIC ANTIDEPRESSANTS, MAO INHIBITORS, QUINIDINE, DISOPYRAMIDE, PROCAINAMIDE, PHENOTHIAZINES, ANTIHISTAMINICS (TERFENADINE) CAN INCREASE THE PRESSOR EFFECT OF SYMPATHOMIMETICS, PROLONG THE QTC INTERVAL AND INCREASE THE RISK OF VENTRICULAR ARRHYTHMIAS. BETA-BLOCKERS WEAKEN THE EFFECT OF FORMOTEROL AND SHOULD BE AVOIDED. CONCOMITANT USE OF OTHER BETA-ADRENERGIC DRUGS CAN HAVE AN ADDITIVE EFFECT.