Addictive is the argument in favor of making it over the counter. Addicts will do whatever it takes to get their fix, so if there isn't a legal path, they buy on the street, creating a funding source for organized crime and spurring gang violence.

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.