Expectorant cough syrups are available under several over-the-counter brand names, including Mucinex and Robitussin, according to Mayo Clinic. The active ingredient providing the expectorant for these medications is guaifenesin. Guaifenesin is also available by prescription.
Guaifenesin works by thinning and loosening mucus in the airways and clearing them to make breathing easier. It treats coughs due to colds, bronchitis and other respiratory illnesses. It is not for use with coughs due to smoking, chronic bronchitis or emphysema, except by a doctor’s direction, warns WebMD. While products containing guaifenesin can make patients with the common cold more comfortable, it does not cure or reduce the time the body requires to get well.