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Any cough more than 2 weeks needs investigation to rule out tuberculosis;bronchitis
and since you say you have got checked i guess you did an X-ray also and nothing is wrong in it.If you don't have fever
and just cough see if you have breathlessness also associated with it.The reason may be a allergic cough
and you will need a antiallergic and even a bronchodilator
if you are having slight difficulty in breathing.Consult a Physician an M.D and get examined.You can also take nebulization from a doctor which will relieve your chest and the cough should get controlled.If you are under treatment and have been properly investigated discuss the use of nebulizers;inhalers for relief from the cough.