Guaifenesin is an expectorant. It helps loosen congestion in your chest and throat, making it easier to cough out through your mouth. Hydrocodone is a narcotic cough suppressant. Guaifenesin and hydrocodone is used to treat cough and reduce chest congestion caused by the common cold, flu, or allergies. Guaifenesin and hydrocodone may also be used for purposes not listed in this medication guide.