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Thank you for using Just Answer. My name is Rebecca; I am a veterinarian. Please give me a moment to type my answer.
I am sorry to hear you are worried about Brad. I see he got an antibiotic, Convenia, which is a very easy way to give a cat antibiotics since it is a 14 day injection, but in my experience is not a great urinary antibiotic. The only way to tell if it is working and the infection is gone would be to do urinalysis.
I see he is taking prazosin, which helps to relax his urethra so urine can pass. I might consider also giving him something for pain and inflammation, such as Onsior, an anti inflammatory medication.
If crystals were seen in his urine, you might ask about starting him on a crystal dissolving diet such as C/D or Urinary S/O.
Let me know if this is helpful and what other questions you have.
Buprenorphine is a good pain medication, but does not treat inflammation. Likely his bladder is inflamed. The food may help dissolve the crystals. You do not have to wait 14 days to have a urinalysis done; you would have to wait that long to do a culture of his urine.
If he is still straining, he may not stop until he receives different treatment. He will stop straining when his bladder is not inflamed and painful, when any spasms in the muscles of his bladder and urethra stop, and/or when the crystals and infection in his bladder are gone.
Please let me know if this was helpful, and what other questions you have, and how he is doing.
I have not heard back from you; I hope you were able to see my response. How is your cat doing?