Guaifenesin is an ingredient in cough and cold medicines that is used to help clear mucus or phlegm from your chest when you have had a cold or flu. Guaifenesin works by making your mucus thinner and easier to cough up, which helps clear chest congestion. Guaifenesin is used to help clear mucus or phlegm (pronounced flem) from the chest when you have congestion from a cold or flu. It works by thinning the mucus or phlegm in the lungs. This medicine is available both over-the-counter (OTC) and with your doctor's prescription. Guaifenesin's neurological properties first became known in the late 1940s. Guaifenesin is a centrally acting muscle relaxant used routinely in large-animal veterinary surgery. Guaifenesin is used to treat coughs and congestion caused by the common cold, bronchitis, and other breathing illnesses. Learn about side effects, drug interactions, dosages, warnings, and more. Guaifenesin is commonly used to treat chest congestion caused by the common cold, flu, bronchitis, and other illnesses. Guaifenesin may also be used for other conditions as determined by your. Guaifenesin is used to relieve chest congestion. Guaifenesin may help control symptoms but does not treat the cause of symptoms or speed recovery. Guaifenesin is in a class of medications called expectorants. It works by thinning the mucus in the air passages to make it easier to cough up the mucus and clear the airways. Guaifenesin is in countless cold medicines, but does it actually work? Here’s what the clinical evidence really says about this common expectorant.