This occurs when a substance passes from the fluid in the tubular lumen through the apical membrane, across the cytosol , and then into the interstitial fluid.
Yes, it's potent stuff. Cat receive it either as an injection or orally. It takes effect in roughly half an hour, lasting about 8 hours before another administration is necessary. Cats usually react negatively to opiates, but buprenorphine is an exception. That's one reason vets use it if they can receive the necessary special certification.
When implemented for a period of three months, the number of patients who relapsed reduced to 50%. These may seem high but in general, no matter what treatment used, addiction carries a relapse rate of 50-90%. If fifty percent is the low end of the spectrum, it seems buprenorphine is pretty effective.