Just say you want Eugenics, because that's whst you really mean by "scientifically valid". Prohibition policies were usually brought in alongside eugenics policies, because the "prohibition" was selectively enforced specifically to protect the eugenic interest.
If you wanted to wipe out the drug trade, you could do so very easily within a generation. It'd be bloody, but far less bloody than the eugenics you've been pointing to. The reason why this doesn't happen is because the authorities quite like the proliferation of drugs, because it reduces the population and dulls their ability to resist eugenic rule. Only a concerted effort to restore actual virtue among the people would be effective in the long term. But, the drug trade, which has always been protected and coddled and is by no means the natural order, that could be stamped out very easily. Civilized countries put down heroin and cocaine dealers like dogs and don't think twice about it. Simply doing that would stamp out the worst of the problem in short order, then you rehabilitate the existing addicts and show a different way of life is possible. Rehabilitation is anathema to everything Eugenics stands for, which is why such an obvious strategy is rejected. But that is the only thing which has ever seriously improved outcomes against the drug culture, or the flesh trade, or any other vice.
Anyway, you could treat the heart conditions caused by smoking and alcoholism very cheaply, even if smoking were as common as it was in the first half of the 20th century. This was not a problem in that time, and life expectancy kept increasing even with a large part of the adult population smoking like chimneys. Usually the treatment plan involved telling the patient to stop smoking, and a decent number of people did exact that without any overbearing manager telling them. There are people every day that quit smoking because it's bad for their health, or because it's too expensive, or because they're sick of being dependent on nicotine. Eugenic society generally tells people there is no escape from vice and addiction, because their entire moral philosophy is premised on addictive behaviors - that's what utilitarianism is, and why they're such abominable lizard-people. That's why Eugenics and neoliberalism advance these peculiar narratives around drugs, glorifying the drug culture and this philosophical idea of pleasure and pain and ignoring the actual people who live through this shit. It was kPost too long. Click here to view the full text.