How do i know if cough of 1.5 months is bacterial or viral? caused from sinus infection, post nasal drip frm a cold.if bacterial, which antibiotic to take? don't want to become resistant to antibiotic

Unlikely: A cough of this duration requires a doctor to listen to your lungs and to take an x- ray to rule out a tissue mass in the lungs.

Not viral: Viral illnesses resolve in 10-14 days so lingering cough could be bacterial subacute sinusitis, residual post nasal drip or airway inflammation. If bacterial Amoxicillin a good first choice. If cough persists then see md. Unlikely to develop resistance with infrequent antibiotic usage.