Mucinex DM helps clear out excess mucus and controls cough. Plus, it lasts for 12 hours. Mucinex DM helps loosen phlegm and thin bronchial secretions to rid the bronchial passageways of bothersome mucus and make coughs more productive. It provides temporary relief of cough due to minor throat and bronchial secretions to rid the bronchial irritation that may occur with the common cold or inhaled irritants, the intensity of coughing and the impulse to cough to help you get to sleep.