The mechanisms of actions of cough medicines are not always known. The problem is exacerbated for herbal medicines, where the effectiveness of the plant or its phytochemicals have rarely been carefully evaluated. Moreover, the most active phytomedicinal constituent is difficult to identify, and the expense and difficulty of such studies discourages sponsors who may not be able to benefit by subsequent exclusive marketing of the herbal remedy. Most popular herbs used as cough medicines appear to be demulcents whose action is confined to the oropharynx. It is probable that the vast majority of allegedly effective herbal cough medicines act as non-specific emetic-expectorants. The proof of activity of even marketed herbal derivatives such as guaifenesin and codeine is difficult to obtain. It is therefore likely that herbal cough medications will never be shown to be more active than placebos. Nevertheless, these plant products will continue to be popular remedies for patients and their health care advisors.