Demulcents are useful for coughs originating above the larynx. They form a protective coating over the irritated pharyngeal mucosa. They are usually given as syrups or lozenges and include acacia, licorice, glycerin, honey, and wild cherry syrups. Humidifying aerosols and steam inhalations exert an antitussive effect by acting as a demulcent and by decreasing the viscosity of bronchial secretions. The efficacy of added medicaments has not been clearly proved.
Most expectorants increase secretions through reflex irritation of the bronchial mucosa.
Some, like the iodides, also act directly on the bronchial secretory cells and are excreted into the respiratory tract. The use of expectorants is highly controversial. No objective experimental data show that any of the available expectorants decreases sputum viscosity or eases expectoration. Data may be lacking partly because of inadequate technology for obtaining such evidence. Thus, the use and choice of expectorants are often based on tradition and the widespread clinical impression that they are effective in some circumstances.