The cough suppressant (also known as an antitussive) dextromethorphan (DM) works by suppressing the cough center of the brain. Basically, you should cough less while taking DM. It is different than other cough medicines called the expectorants, such as guaifenesin. Expectorants only thin out the phlegm or mucus, making coughing easier, but not suppressing cough like DM. Conversely, DM suppresses the nervous system and coughing center in the brain. DM does not thin the mucus like the expectorants. DM is commonly combined with expectorants to form a more complete cough medication.