BENYLIN® Chesty Coughs Original provides deep penetrating relief and a soothing effect that allows you to get on with your day How it works: Active ingredient diphenhydramine hydrochloride is an antihistamine and works by relieving a cough. Diphenhydramine also helps dry nasal secretions by preventing the natural body chemical histamine from binding to its receptors and producing a runny nose. Levomenthol works to relieve stuffiness in the nose to help make breathing easier. What is a chesty cough? Chesty coughs are caused by a build-up of phlegm in the lungs and on the chest. The cough mechanism kicks in to try and get rid of this excess mucus.