Reduced effect of formoterol w/ β-blockers. Additive effects of formoterol w/ theophylline or other β-adrenergic drugs. Prolonged QTc-interval & increased risk of ventricular arrhythmias w/ quinidine, disopyramide, procainamide, phenothiazines, antihistamines, MAOIs, TCAs. Impaired cardiac tolerance w/ L-dopa, L-thyroxine, oxytocin, alcohol. May precipitate hypertensive reactions w/ furazolidone, procarbazine. Hypokalaemic effect w/ xanthine derivatives, steroids, diuretics. Disulfiram, metronidazole.