Method for relieving cough using an effective antitussive chemical compound belonging to stemoamide or tuberostemospironine and method for manufacturing pharmaceutical compositions for treating or relieving cough in human and animal subjects which including a stemoamide or tuberostemospironine compound. In addition, the antitussive property of the compounds provides a method for assessing the quality of herbs traditionally used in treating cough by analyzing the content of the effective chemical ingredient, i.e., compounds of stemoamide type and/or tuterostemospironine type. It further provides a method for identifying medicinal herbs which may be used for relieving cough by phytochemical determination of the existence of compounds that are of stemoamide or tuterostemospironine type.