The common side effects of Mucinex DM include stomach upset, dizziness and drowsiness. A person may also experience severe side effects, such as breathing difficulties, itching, rash, swelling and chest tightness.
Mucinex DM is a combination of drugs that is use to treat coughs that are caused by bronchitis and common colds. Severe side effects while using this drug indicate that there are allergic reactions, which require medical attention. Not all side effects of the medicine are usually reported, so a person should visit a doctor in the case he or she develops a different condition while using the drug. The common side effects do not indicate any serious problem to make one stop their dose, as stated by WebMD.