The belief that milk might increase the production of mucus has been around for centuries. However, this seems to be more myth than fact. Experts believe that although drinking milk may make phlegm thicker and possibly more irritating to your throat than it would normally be, it doesn’t cause your body to make more of it or increase nasal secretions. So don’t worry about enjoying a cool glass of milk or a frozen dairy product when you are sick and have a sore throat.