Hi!
Can you identify any triggers for your UTIs? Unfortunately sex is a definite trigger for me, despite taking all the usual post-coital precautions (good hygeine, passing urine straight afterwards...).
I now have few problems as my GP prescribes me preventive Trimethoprim. I take a 200mg tablet just before or after sex and it stops any bugs in their tracks.
You will see that Trimethoprim is listed as a drug that can interact with Azathioprine. My friend is a pharmacist and she has investigated this and found that it is only a problem for patients on higher doses of Aza (transplant patients) and who are generally unwell. We also figured that the occasional (and it is very occasional - ha! ha!) 200mg is better than regularly ending up needing a few days course of the stuff.
Maybe a diary could help you find any links - sex, swimming, any foods/drinks, other activities.
Also, (although I'm not a doctor and we all have different problems with different drugs), if you do respond to it you might find trimethoprim better than other antibiotics as it does not affect the whole body like other antibiotics and so is less likely to interfere with the bug balance of other areas of the body - gut, mouth, etc (I am also prone to thrush and Trimethoprim doesn't cause this in me). Anyone medical out there might be able to put me right on that if I'm wrong.
Good luck.