DM in cough syrup stands for dextromethorphan, which is a cough suppressant. It works by acting directly on the cough center in the brain, reducing the urge to cough. Cough syrups containing DM are used for temporary relief of coughs without phlegm that are caused by certain infections of the air passages, such as sinusitis or the common cold. It is important to carefully follow all dosage directions and not use these products to make a child sleepy or give other cough-and-cold medication that might contain the same or similar ingredients. It is also important to note that cough-and-cold products have not been shown to be safe or effective in children younger than 6 years.