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.
For a drug like Suboxone, however, the half-life is far longer. A combination of buprenorphine and naloxone, buprenorphine has a half-life ranging from 24 to 42 hours, while naloxone has a half-life of two to 12 hours. Patients are often directed to take their dose once every two to three days in order to maintain the correct levels of the medication in the system.