Drugs is a voluminous issue, and a complicated one, as many things go into effective regulation (neighbouring countries a large part, and so are follow-up and auxiliary policy). I could just say "i draw the line at being an unnecessary risk for uninvolved people". Portugal has a successful drug policy, but it only works *because* they also go the extra mile to supply clean needles to drug addicts. Otherwise you'd have a problem with STDs and HIV.
But you get similar results with the complete opposite policy, like in south korea, singapore or japan. There is still an underground scene, no doubt, but the prevalence of drug use or drug related crime is very low.