Not "Legalization" which implies that you can obtain drugs through vending machines and pharmacies...think "De-Criminalization".
As stated, the addict receives their drug at a clinic and the only way it leaves the clinic is in the addict's body. Education and rehabilitation is also available there. They are registered with the clinic which carries with it the inability to get a job in an industry where we currently test for drug use. A kid couldn't just stop in to sample the drugs to see what they favor, unlike our current system of obtaining drugs on the ghetto street corner.
Insanity is keeping on doing what you are doing and expecting a different outcome.