Concomitant use of antihistamines with alcohol, tricyclic antidepressants, barbiturates, or other CNS depressants may have an additive effect. The action of oral anticoagulants may be inhibited by antihistamines. When sympathomimetic drugs are given to patients receiving MAO inhibitors, hypertensive reactions, including hypertensive crises, may occur. 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.
Patients should be warned about engaging in activities requiring mental alertness, such as driving a car or operating appliances, or machinery. Antihistamines are more likely to cause dizziness, sedation, and hypotension in patients over 60 years of age. In these patients, sympathomimetics are also more likely to cause adverse reactions, such as confusion, hallucinations, convulsions, CNS depression and death. Overdosage of antihistamines, particularly in infants and children may produce convulsions and death. Antihistamines should be discontinued approximately 4 days prior to skin testing procedures since these may prevent or diminish otherwise positive reactions to dermal reactivity indicators.