Hi.. Thanks for the query.. I have gone through your query and can understand your concerns.. As per your complain the symptoms of severe cough and phlegm production along with Post nasal drip and breathlessness can occur due to a number of causes like severe upper respiratory tract infection, bronchitis, lung infection, chronic obstructive pulmonary disease etc.. I would suggest you to consult a Pulmonologist and get evaluated and a thorough clinical evaluation and investigations like sputum culture, chest x ray and complete blood picture can help in confirmation of cause and treatment can be planned accordingly.. Mucinex DM contains Dextromethorphan that is a cough suppressant while Gueifenesin is cough expectorant and will help in loosening the mucous and ease in drainage and it will be helpful.. But in case if there is infection the you need to get it treated to relieve the symptoms.. You can be advised a course of antibiotics like Cephalexin, anti-inflammatory painkiller like Motrin, anti allergics like Levocetrizine, decongestants like Phenylephrine, mucolytics like Ambroxol, cough syrups and also warm saline gargles, steam inhalations and used humidifier in your room.. Avoid spicy and chilled foods and drink warm water.. Take ample of rest.. Hope this information helps.. Thanks and regards. Dr.Honey Nandwani Arora.