Benylin Chesty Non Drowsy 300ml helps relieve chesty coughs by loosening and thinning phlegm in your chest. The active ingredient guaifenesin makes it easier to cough up excess mucus, while levomenthol helps relieve congestion, making it simpler to breathe. A chesty cough occurs when there's a build-up of phlegm in the lungs and on the chest, triggering the cough mechanism to try and get rid of it.