Hi, Mucinex contains guaifenesin which is an expectorant. It is used to treat chest congestion and cough. It breaks up the mucus that is stuck in the airways so one can cough it up. This way it helps relieve chest congestion. Take this medication by mouth with or without foods, as directed by your doctor. Usually every 12 hours with a full glass of water. If you are self treating, follow all directions on the product package. Side effects on your skin are usually unusual at a normal dosage of Mucinex, but may include an allergic reaction. This reaction typically causes a rash or raised splotches on your any part of skin like chest. If you have a skin rash after using Mucinex, stop taking this medication and contact your doctor promptly. Mucinex may cause other symptoms of allergic reactions like itching, swelling of lips, tongue, throat, face, eyes, hands or feet, wheezing, breathing difficulties, dizziness, rapid heartbeat etc. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Zubayer Alam, General And Family Physician