The letters "DM" in cough syrup are an abbreviation of dextromethorphan, which is a cough suppressant that belongs to a group of drugs called antitussives. It works in the brain to reduce the desire to cough.
Dextromethorphan is used to provide relief on a short-term basis for coughing caused by bronchitis and the common cold. A person who is allergic to Dextromethorphan should not use any drug that contains it. Those taking monoamine oxidase inhibitors and people suffering from respiratory depression should also avoid using this medication, as stated by Canada. This drug should also not be used for children less than 6 years of age without consulting a doctor.