Mucinex (guaifenesin) is used to relieve the symptoms of cough and mucus in the chest due to colds, flu, or hay fever. Mucinex may also be used for other conditions as determined by your doctor. Mucinex is an expectorant. It works by thinning mucus (phlegm) in the lungs and making it less sticky and easier to cough up. This reduces chest congestion by making coughs more productive.