Concomitant or recent (previous fourteen days) monoamine oxidase inhibitor use can lead to hypertensive reactions, including hypertensive crises.
The antihypertensive effects of methyldopa, mecamylamine, reserpine, and veratrum alkaloids may be reduced by sympathomimetics. Beta-adrenergic antagonists may also interact with sympathomimetics. Increase of ectopic pacemaker activity can occur when pseudoephedrine is used concomitantly with digitalis. Antacids increase the rate of pseudoephedrine absorption, while kaolin decreases it.