Hello, Dextromethorphan is a cough suppressant. Cough that is caused by smoking, asthma, emphysema, COPD or any underlying medical condition cannot be treated with Dextromethorphan. Get clinically examined by a GP, get sputum test, blood tests, X-ray, CT, MRI lungs done to detect the pathology causing you persistent cough. If the test reports come positive you will be treated for that specific medical condition that needs to be treated. Other medical conditions like sinusitis, postnasal drip, deviated nasal septum may trigger allergy like symptoms. Get evaluated by an ENT and get X-ray PNS done to rule out deviated nasal septum. Postnasal drip is commonly treated with decongestants, antibiotics (if bacterial infection), nasal sprays, nasal saline irrigation. Telfast-D will help to control allergy like symptoms and help to give relief from coughing. Take salbutamol as it will open up your airways and prevent breathlessness. Keep cough lozenges in your mouth so that the throat does not dry, dryness of the throat mucosa will trigger cough. Talk less, take sips of water throughout the day. Stay well hydrated with fluids and water. You are suggested to take liquid food or semisolid food so that your throat mucosa is not hurt. Take care. Hope I have answered your question. Let me know if I can assist you further. Regards, Dr. Nupur K., General & Family Physician