Thank you so much for your response! You have just touched on what I’m so confused about. What do you mean by hit? To me that conjures a significant difference in feeling. I just feel nothing. At 8 mg in an injection shouldn’t there be a noticeable boost? If not, this is the essence of my question, why inject it at all? Are they doing it strictly because of the strong BA difference?
"Hit"? Basically, how fast the effects come on.
What is your daily dose of bupe?
As I said, buprenorphine is a partial agonist, and as such has a "ceiling", where even major increases in dosage barely increase the intensity of the felt effects.
Buprenorphine has an oral bioavailability of ~30%, so IV'ing it more than triples what you can get into your system. But if 8mg of buprenorphine is your normal (sublingual) dose, you're already pretty close to the effects ceiling. So even if, by IV'ing, you're essentially turning those 8 mg into the equivalent of ~25 mg, it's not going to matter much, because 25 mg of buprenorphine doesn't feel much stronger than 8.
Also, buprenorphine has a very long half-life. If you take 8 mg of buprenorphine on a daily basis, you'll have so much bupe left over in your system that even the massive amount of buprenorphine you're getting into the bloodstream all at once isn't going to make much of a difference because the buprenorphine that's still left over from your last dose is enough to keep your opioid receptors activated as strongly as it gets with bupe.
On the other hand, the situation is different for someone who's getting a low dose of subutex, where there's still plenty of unoccupied receptors for the bupe to bind to. Here, the dose-response curve is still somewhat linear, so iv'ing 2 mg's of buprenorphine to effectively triple the bioavailability is going to make a significant difference in felt effect.