There are provided novel antitussive agents different from existing antitussive agents or those under development. Antitussive agents containing morphinan derivatives represented by compound 6: ##STR1## or pharmacologically acceptable acid-addition salts thereof and those derivatives or their salts as effective components. The antitussive agents according to the present invention have strong antitussive effects as highly selective .kappa.-opioid agonists, and may thus be applied as useful antitussive agents in the prophylaxis and treatment of coughing in a patient in need thereof.