Expectorants are a class of cough medication that help thin mucous. Once mucous is thinned, it is easier to expel from the lungs by coughing.
The only over-the-counter expectorant is called guaifenesin. It is sold under multiple brand names, including:
Expectorants may be useful if you have mucous in your chest that you are having difficulty coughing up. However, it should not be used for a cough caused by COPD, asthma, smoking, emphysema or chronic bronchitis.