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.
Tell your doctor if buprenorphine does not relieve your pain within 1 hour after an injection. Prepare your dose in a syringe or IV only when you are ready to give yourself an injection. Do not use the medication if it has changed colors or has particles in it. Call your doctor for a new prescription.