Concomitant use of antihistamines with alcohol , tricyclic antidepressants, barbiturates, or other central nervous system depressants may have an additive effect. When sympathomimetic drugs are given to patients receiving monoamine oxidase inhibitors, hypertensive reactions, including hypertensive crises, may occur. The antihypertensive effects of methyldopa, mecamylamine, reserpine, and veratrum alkaloids may be reduced by sympathomimetics. Beta-adrenergic blocking agents may also interact with sympathomimetics. Increased ectopic pacemaker activity can occur when pseudoephedrine is used concomitantly with digitalis.
Antacids increase the rate of absorption of pseudoephedrine, while kaolin decreases it.