@The Holy Quadruplty if we are talking about drug facts that "even doctors don't know", I can pretty much assure you that we are that. You won't find a better resource for those answers than us right here. The BL'ers that have made us what we are today did so by collecting and validating information over the course of years... decades now. I'm not sure if you've ever been over to Advanced Drug Discussion, but some of those folks are fuckin crazy. The most-knowledgeable BL'ers in that forum would look at a thread like this as 2nd or 3rd grade level stuff. I'm not putting you down. I want you to know what we have here. If you have the questions, there are people here with the answers if you look hard enough.
There are several substances out there with these similar, I guess, auxiliary qualities. These substances are first and foremost Opioids, though they have auxiliary effects which can also play a significant role in their effects. To name a few:
Tramadol - Mu Opioid agonist/Serotonin-Norepinephrine Reuptake Inhibitor (SNRI)
Tapentadol - Mu Opioid agonist/Norepinephrine Reuptake Inhibitor
Methadone - Mu Opioid agonist/N-Methyl-D-Aspartate (NMDA) Antagonist
Buprenorphine - Mu Opioid partial agonist/Delta + Kappa receptor antagonist at higher dosages/NMDA antagonist
What will happen in the course of prescribed usage, is that the physician will identify that the patient needs Opioid therapy. It might be determined that one of these medications has the potential for a small "bonus" if you will to the treatment based upon that patient's specific needs. Also, these medications might be chosen without any attention paid to their auxiliary effects.
But yes, I'll definitely agree with you that Buprenorphine is a comparatively complex drug. While the Mu receptor is the straightforward, easy to understand receptor, the Kappa and Delta receptor modulate stuff other than pain sensitivity.
Buprenorphine is a drug you can spend a lot of time learning about so enjoy.