About Dextromethorphan A synthetic morphine (((dextrorotatory methyl ether of Levorphanol))) derivative,A synthetic narcotic antitussive. Mechanism of Action of Dextromethorphan Dextromethorphan hydrobromide is a selective antitussive which raises threshold of cough centre and suppresses the cough reflex by direct action on the cough centre in the medulla. It does not depress mucociliary function of the airway mucosa and is also devoid of constipating and addicting actions. (((Its antitussive action is not exerted through opioid receptors))) Indications for Dextromethorphan Chronic nonproductive cough.