I Have Re-occurring Bladder Infections Sine The Age Of Twenty

I have re-occurring bladder infections sine the age of twenty two, l am now just 70 years old, l have had had all the tests and they can't find out why l keep getting them, l have just got over another severe attack and the feeling of passing razor blades when urinating, l have been on another course of anti-biotic that stop the pain and get me back to as near normal as possible with my bladder, but that takes at least two weeks after an infection, l am going for another scan this Wednesday, I have ask my GP for low dosage anti-biotic to take when an attack occurs (this was suggested by the Oncologist) but he is reluctant to do so!!!! what do you think, l desperately need some treatment to stop me getting these attacks and feel depressed there is no long term answer. Thank you.