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.
If you do stop buprenorphine, expect withdrawal to peak at about 4-7 days after you finally discontinue taking Suboxone, followed by slow recovery that accelerates each week. By four weeks, you will be done with the creepy crawly legs, and your energy will be starting to return.