https://www.aei.org/carpe-diem/what-does-science-tell-us-abo...
Personally, I think a great amount of harm reduction could be achieved by legalizing at least one recreational drug in every class, determined by which has least relative harm to the user. If there was a clean, legal and affordable option, I believe demand for the more-harmful drugs in the class would be significantly reduced. i.e. "stimulant" demand is somewhat elastic, and by making the least harmful stimulant (not sure if that is Khat or not) easily available, demand for Meth would be significantly reduced and market effects would make its availability much less widespread
Psilocybin is the first possible candidate that comes to my mind, but I'd rather have us use a sound framework of harm-minimization than to favor/disfavor any particular drug.