Official answer by Drugs.com Mucinex D is (guaifenesin and pseudoephedrine) is an expectorant/nasal decongestant combination, and Mucinex DM (guaifenesin and dextromethorphan) is an expectorant/cough suppressant combination. Mucinex D and Mucinex DM both contain the expectorant guaifenesin (which is contained as a single ingredient in Mucinex ). Mucinex and Mucinex DM both contain an expectorant that helps loosen mucus. Mucinex DM BSh3.