There are many reasons for an old cough, which may be related to allergies, infections, side effects of medications, psychological factors and so on. 1. Allergy: Coughing is one of the body’s self-protection mechanisms, such as inhalation of allergens, the stimulation of foreign substances, can trigger the body’s response, resulting in patients with old cough. The purpose of coughing is mainly to expel foreign bodies in the respiratory tract or secretions produced by the respiratory tract. 2. Infection: pathological factors may also cause cough, such as acute bronchitis, upper respiratory tract infection, chronic bronchitis, chronic obstructive pulmonary disease, asthma, etc., the respiratory tract secretion increases, the local stimulation can cause old cough. 3. Side effects of drugs: some patients cough may be caused by the side effects of drugs, such as taking captopril and other antihypertensive drugs, may cause old cough. 4. Psychological factors: Psychological factors such as stress, anxiety and depression may also cause old cough. There are many reasons for the old cough, should go to the regular hospital in time, clear cause, under the guidance of the doctor for treatment.