Hello As you mentioned in your history of illness that , you are a patient of asthma . Cough non-stop for three months. Lungs are O K , taking seritide and ventolin . In my opinion you are asthmatic patient and in asthma there develop bronchoconstriction . While bronchoconstriction causes bronchospasm and this produces cough . You are taking seretide and ventolin , they are respectively acting as antihistamine and bronchodilator but in your case ventolin is not effective , so your cough is not responding to this treatment . In my opinion you require a more potent bronchodilator . In such type of cases I prescribe monteleukast + bambuterol one tablet twice in day till complete recovery . Beside this steam inhalation will also provide relief. You should also take codeine+ chopheniramine maleate syrup according to necessity . In my opinion these measures will provide relief. Hope this will help.