This combination medication is used to treat symptoms caused by the common cold, flu, allergies, hay fever, or other breathing illnesses (e.g., sinusitis, bronchitis). Dextromethorphan is a cough suppressant that affects a certain part of the brain (cough center), reducing the urge to cough. Decongestants help relieve stuffy nose symptoms. Antihistamines relieve watery eyes, itchy eyes/nose/throat, runny nose, and sneezing.

Source: http://www.healthsquare.com/drugs/101940.htm