It encourages antibiotic resistance because the susciptible bacteria are destroyed but the resistant ones survive and multiply, leaving only resistant ones. An increased dose may be required to achieve a succesful treatment but all too often is just another stpe in having even more resistant bacteria, ultimately, the antibiotic may become useless - and not just for that patient but in the wide community because the resistant strain infects other people and can pass the resistance factor to other bugs also.
There is already widespread resistance to first line treatments like trimethaprim and worrying it is starting to develop in second line treatments, even "strong" antibiotics like carbapanems. It is even starting to appear against drugs of last resort - after which no further treatment is available.
Long term antibiotic treatment can also result in side effects, espescially if the drug happens to also kill off the helpful bacteria that normally reside in our digestive systems. This can lead to things like malabsorption of nutrients or more severe problems such as recurring diarrhea.
Most doctors are resistant to long-term antibiotic use for these reasons.
Cranberry juice can work quite well. Cranberry extract tablets can be used if the juice is not suitable for some reason. Good hydration is vital - essentially this means that the irnary system is regularly (and literally) flushed clean.
Without good hydration urine becomes concentrated and stale, and the urinary system is only "flushed" irregularly. It becomes an ideal home for bugs.
