Antitussive drugs. II. Bronchopulmonary effects of dextromethorphan and doxylamine

Unlike codeine, dextromethorphan does not cause pulmonary vasoconstiction in the anesthetized dog. The other effects of dextromethorphan are as follwos: a rise in pulmonary resistance indicating bronchoconstriction, and increase in pulmonary blood flow relating to a positive inotropic action, and a decrease in aortic blood pressure resulting from systemic vasodilation. These effects are blocked by previous injections of doxylamine, an antihistaminic drug. This interaction suggests a rationale for the combined use of dextromethorphan and doxylamine in the treatment of upper respiratory infection.