BENYLIN® Chesty Coughs Non-Drowsy works deep down in the chest to help loosen phlegm, so that when you cough you can actually cough some of the phlegm up. This helps clear bronchial congestion. How it works: Active ingredient guaifenesin helps thin and loosen phlegm in your chest, making it easier to cough up. The active ingredient levomenthol also helps to relieve congestion to make it easier to breathe. 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 phlegm.