However after seeing how business behaves when it can sell opiates, I'm in favor of even more regulation than we currently have on some drug markets
(Also, if it were up to me, I'd ban ads for drugs, including alcohol and prescriptions)
Not just because it results in drugs getting cut with all kinds of shit, and people being sold the wrong thing (e.g. selling heavily cut fentanyl instead of heroin), but also because of a the race to find alternatives that slip through cracks.
A great deal of modern drugs only exists to circumvent the law, either because they're not covered for a while, or because they can more easily be produced. E.g. a number of "synthetic cannabinoids" have been manufactured, and so far indications are that at least some of them have health effects that are far worse than anything possible to tie to actual cannabis.
Without decriminalising and regulating manufacture and sale, there will still be an incentive for dealers to sell drugs like that which nobody particularly want, and that increases harm, instead of selling clean versions of the safest drugs.
E.g. for opiates, many of them are "close enough" in terms of effect that there is a lot of potential harm reduction benefit just in getting the more dangerous variants off the market by legalizing and regulating the safest ones. Unlike blanket bans that has a hope of working.
For opiates, especially people injecting, it is very inportant to get clean drugs, and actually opiates are not particularly harmful to the body if used correctly. They also don’t really present a threat to society if given the drug. Holland has provided addicts with heroin with great success, as a form of treatment.
Ultimately I think that we'd in many respects do a lot better if it was possible for someone who wants to use things like Adderall for recreational use to go to their doctor and ask for advice and appropriate monitoring and know they won't be refused, than having people randomly taking it without getting proper advice.
But I also think another large potential benefit would be for doctors to be able to steer those who insist on using drugs to safer analogs were possible.