Buprenex and Cats

Our Elizabeth Taylor and Bette Davis have been on Buprenex since last spring for idiopathic cystisis. It's helped but lately, in December, they developed an inability to urinate often enough. I've tried adjusting the med up and down. In January, Bette began going out of the box about once or twice a day which turned out to be all that she could release.  She doesn't have a UTI or there would be other indications. If she has crystals, there's nothing to do but what we are doing-feeding her water by mouth, in the food, feeding wet SO, etc.  So, I've cut back on the med to twice a day and will wean her off down to .1 ml or even off of it to see how she does. I want the bladder to function properly again. I surmise that the accidents are due to the bladder being too full and she not being comfortable enough and so she goes out of the box.  Lizzie is doing fine on less meds and going more appropriately. A known side effect of Buprenex is urine retention.