1. Mucinex D helps with nasal congestion, while Mucinex DM helps quiet a cough Both Mucinex D and Mucinex DM help relieve chest congestion. This is because they both contain guaifenesin (an expectorant ). It helps relieve chest congestion by thinning and loosening mucus in your lungs and throat. Fact Checked Mucinex DM is an over-the-counter (OTC) combination medication that relieves cough and chest congestion caused by the common cold, flu, and allergies. Each tablet combines dextromethorphan hydrobromide, a cough suppressant, with guaifenesin, a drug that loosens mucus and bronchial secretions in the lungs. 4vxtU2.