From Robitussin
This medicine contains the active ingredient guaifenesin. This belongs to a group of medicines called cough expectorants that work by changing an unproductive cough to a cough that is more productive and less frequent1. Symptoms of a chesty cough: Productive: thick and sticky mucus or phlegm. Tightness in the chest.2. How Robitussin works: Loosens and thins mucus or phlegm making it easier to cough up. Soothing effect.