Benylin Dry Coughs Syrup helps to relieve dry coughs by suppressing the cough mechanism. It contains Dextromethorphan, a cough suppressant, and Levomenthol, a decongestant that relieves stuffiness in the nose, making breathing easier. Diphenhydramine is also present, which helps to relieve a runny nose. A dry cough is a non-productive cough that doesn't produce any phlegm, often caused by an inflammation of the upper airways due to dust, foreign bodies, or an infection.