Hi.. Welcome to HEALTHCARE MAGIC.. I have gone through your query and can understand your concerns. As per your complain excessive phlegm build up at the back of the throat can be due to a number of causes like sinusitis,upper respiratory infection, acid reflux, throat infection etc.. Coating over the tongue can be due to yeast infection known as Thrush or can be due to other causes like poor hygiene leading to accumulation of debris.. Bad breath can also be due to sinus infection known as sinusitis as well as other conditions that I have advised above.. I would suggest you to consult an Oral Physician and get evaluated and a thorough clinical evaluation can help in diagnosis and treatment can be planned accordingly.. As of now you can do warm saline gargles, chew flavoured sugarfree chewing gums and gargle with flavoured mouthwash to mask bad breath.. Do steam inhalations with eucalyptus oil drops in boiling water.. Maintain a good oral hygiene.. Hope this informations helps.. Thanks and regards. Dr.Honey Nandwani Arora..