hi there! coughing is actually a normal physiologic function of our throat to expell all irritants in the upper airway. irritants could be allergens, smoking, virus, or bacteria. there are lots of factors to consider if you ve been coughing for 3 weeks already. does your cough had phlegm? what is the color of your phlegm? do you have colds or coryza? do you have post nasal drip? do you have fever? do you smoke? do you have allergy and had been eating poultry products, crustacean, tuna, eggplant? does your cough goes away when your home and you just cough at your workplace? are you exposed to dust? do have bronchial asthma? we have different causes of coughing: allergic cough, asthma, chronic obstructive pulmonary disease, bronchitis, pneumonia, etc. subject yourself for chest xray to evaluate your lung, get a complete blood count to evaluate the presence of infection (viral/ bacterial), avoidance of allergens and a pulmonologist for evaluation of your lung status. so that you will be given the right medicine.