Mucinex tablet is an expectorant that contains Guaifenesin. It helps loosen congestion in your chest and throat, making it easier to cough out through your mouth. Mucinex is used to reduce chest congestion caused by the common cold, infections, or allergies. A unique bi-layer tablet, one layer for immediate release, the other layer for extended release so the medicine starts to release right away and lasts for up to 12 hours.