What is Guaifenesin? Guaifenesin is th etrademark name for a drug known as Mucinex. Guaifenesin is used to treat coughs and congestion caused by the common cold, bronchitis, and other breathing illnesses. This product is usually not used for ongoing cough from smoking or long-term breathing problems (such as chronic bronchitis, emphysema) unless directed by your doctor. Guaifenesin is an expectorant. It works by thinning and loosening mucus in the airways, clearing congestion, and making breathing easier.