>There would still be the social cost, though, of the much more highly-addictive drugs (compared to alcohol).
You're assuming usage would increase. Legalizing drugs but taxing the dickens out of them removes the huge profit motive from distributors, so you would see less "marketing" and fewer unsuspecting people getting addicted in the first place, and licensed distributors would be far less likely to unlawfully target children. Then you can use the tax revenue generated to fund anti-drug programs like the ones that have been relatively successful in reducing cigarette smoking in recent years, to stigmatize drugs use as much as possible socially.
And we can make getting drugs very unsexy: Require them to be sold only by licensed pharmacists who are obligated to describe the correct dosage and side effects etc. For hard drugs, require them to be consumed on the seller's premises with a physician present at all times and the user to be attached to medical monitoring equipment.
The goal should be to make drug use legal but lame. The problem now is that people don't recognize the risks and costs and foolishly choose to consume dangerous and addictive substances. Make them go talk to a legitimate physician in a lab coat who explains how doing this will cause their teeth to fall out and suddenly the whole idea just seems silly. Meanwhile the availability of legal drugs makes it totally irrational to make illegal street purchases and defeats the black market.