Cough, lasting for more than three weeks would be classified as chronic cough.
There can be various reasons for a long lasting cough:
The most common being post, viral infections, others being gastro-oesophageal, reflux disease, and allergic cough.
Other causes of chronic cough are obstructive, lung disease, diseases like COPD, asthma, interstitial lung disease, lung infections, tuberculosis
Cardiac conditions like pulmonary edema, valvular disorders may also cause long standing cough.
Lung malignancies- primary or secondary may also lead to chronic cough.
Warning signs of lung cancer include chronic cough, blood in sputum, weight loss, loss of appetitePrevious history of smoking, malignancy and family history of malignancy.
So consult your pulmonologist at the earliest for diagnosis and appropriate management.