My doctor recommended it during my last check-up (April). My CT scan showed mucus plugs developing. When mucus plugs develop and cannot be loosened and coughed out, then a bronchoscopy will be necessary to remove them. My doctor wants to do as little invasive procedures as possible. I now take 600mg Mucinex twice a day and cough out mucus daily. So, yes, Mucinex does help with phlegm. Take care and God Bless!