The best cough suppressants for a dry cough are those that contain dextromethorphan, according to WebMD. A survey of pharmacists reported in U.S. News & World Report chose over-the-counter cough syrup Delsym as the most effective, with Robitussin coming next in line.
While cough suppressants containing dextromethorphan work well with dry coughs, they are less effective at treating a productive cough that brings up mucus, and they should not be taken with certain other medications. Dextromethorphan reacts with many types of antidepressants. Patients with heart disease or high blood pressure should make sure any cough suppressant containing dextromethorphan doesn't also contain decongestants, which can increase blood pressure levels, as WebMD points out.