There is no thriving black market for antibiotics, because they are accessible when you need them (and most people need them very infrequently for a short duration). In contrast, I believe there is a black market for insulin in the US, and that's because of how ridiculously expensive it is. Exuberant pricing is a form of restricting access, too.
You want to control the general population’s access to physically-addictive substances to control addiction. Managing addicts is not a pharmaceutical matter.
Just yesterday on Reddit there was a thread that went viral for "cutter" reviews on Shein razor blades, with cutesy language like "beautiful beans for my followers" (referring to subcutaneous dermal appearance when deeply cut open). Every product can be abused in horrible ways. It's the nature of the stochastic bubble we're in. People will find every nook and cranny of the human experience.
You can't stop this stuff from happening. So at least let the normal use cases that benefit society through. Don't put everyone else in the same straight jacket. We don't deserve to be punished for the bad gradients some people fall into.
Then, because the black market is already in violation of the law, there are no purity standards. The customer who thinks they're getting Adderall or codeine is actually getting fentanyl because fentanyl's much higher potency makes it easier to smuggle, but for the same reason makes it much more prone to addiction and overdose, especially when careless street dealers get their proportions wrong. All of which is avoided if you just let them buy it from the pharmacy.
Notice that there is no thriving black market for antibiotics propping up international drug cartels, because they're not addictive.