Proper drug education would likely go a very long way towards ameliorating addiction problems (the main contributor to negative externalities in drug markets).

Why are people using heroin? Are they depressed? In pain? What about amphetamines? Do they have ADHD?

Moreover, how many first time users actually know how to/understand the importance of properly titrating up, or even safe dosages of the substance they're using?

Not to mention that people taking substances for underlying mental issues might finally appear on the radar of their GP.