Thanks for your query.
I suggest you to have that zerotuss cough syrup for some days and if cough subsides then its ok but if it persists you should consult a pulmonologist
There are many possibilities of this persistent cough like
upper airway cough syndrome, tuberculosis
, lung cancer specially in a aged smoker.
Therefore these causes are needed to be excluded by proper physical examination and by some investigations.
So you need to consult a pulmonologist if the cough persists even after a 2 weeks course of anti-tussive.