Other centrally acting antitussives include chlophedianol, levopropoxyphene, and noscapine in the nonnarcotic group and hydrocodone, hydromorphone, methadone, and morphine in the narcotic group. Peripherally acting antitussives may act on either the afferent or the efferent side of the cough reflex. On the afferent side, an antitussive may reduce the input of stimuli by acting as a mild analgesic or anesthetic on the respiratory mucosa, by modifying the output and viscosity of the respiratory tract fluid, or by relaxing the smooth muscle of the bronchi in the presence of bronchospasm. On the efferent side, an antitussive may make secretions easier to cough up by increasing the efficiency of the cough mechanism. Peripherally acting agents are grouped as demulcents, local anesthetics, and humidifying aerosols and steam inhalations.
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.