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I am sorry to hear about Simba having cancer. My name is Rebecca. I am a veterinarian and will do my best to help.
This is probably not due to the medication. It could be that he has an infection in his bladder that is is not sensitive to the Orbax, so trying another antibiotic might help. However, it is more likely that he is not able to control his urination due to pain and inflammation from the tumor in his bladder.
You might consider trying piroxicam, a prescription anti inflammatory that has effects against bladder cancer, and in some cases can shrink the tumor for awhile. It can't be given with prednisone, so you would have to stop the pred first.
Let me know what other questions I can answer.
I hope you were able to see my answer. How is Simba doing? Can I help with or answer anything else; was my answer helpful?