Mucinex D is an extended-release of the combination of guaifenesin and pseudoephedrine. Guaifenesin is the most commonly used expectorant that helps you loosen your throat and chest congestion. Pseudoephedrine works specifically in the oral/nasal mucosa by vasoconstricting the dilated blood vessels that cause localized congestion. Do not use it if you are using MAO inhibitors to avoid serious life-threatening side effects. Remember that you don’t need to crush, chew or break the extended-release tablet. Drink plenty of water for the whole tablet and get medical assistance if severe dizziness, anxiety, restlessness, or nervousness occurs. Tell the doctor if you are taking antihypertensives or anti-depressive medication.