hi there, the body doesn't become resistant to a drug rather the infecting organism becomes resistant to it so that they are not killed ny that particular antibiotic and so last time you were given ciprofloxacin
as that particular strain of bacteria was resistant to amox. This time the infecting organism may not be resistand to amox and so your doctor has decided to try it again as it is one of the safest broad spectrum antibiotics working on gram positive organisms which are the most common cause of uppper respiratory infection. Hope its works this time or else it may be changed to ciprofloxacin. Take care.