Mucinex D extended release tablets contain a combination of guaifenesin and pseudoephedrine. Guaifenesin is an expectorant. It helps loosen congestion in your chest and throat, making it easier to cough out through your mouth. Pseudoephedrine is a decongestant that shrinks blood vessels in the nasal passages. Dilated blood vessels can cause nasal congestion (stuffy nose). Mucinex D tablets are used to treat nasal and sinus congestion. Mucinex D also is used to reduce chest congestion caused by the common cold, infections, or allergies.