It's all supply and demand. it depends on the events occurring after it is criminalized or decriminalized; the level of enforcement and more.
Yes, and we can look at concrete data, at least when it comes to addiction.
I was involved in the pro cannabis liberalization and drug consumption decriminalization movement 13 years ago in my country. I grew up before decriminalization. I grew up seeing junkies in every corner, I played near used needles and syringes, cases of addiction in family, neighbors... Practically everybody new someone close with heroin addiction.
13 years after decriminalization things are much better, there are syringe exchange programs, rehabilitation programs, everybody caught using something had to have a few meetings with a psychiatrist, several of my friends had to go, fortunately, I was never caught, and it was always my opinion that send 20 year old college guys who smoke joints to psychiatric consultations a waste of resources, many people with serious addiction could use the attention.
On the other hand, we have US War on Drugs, and has far as I know drugs are winning big time.
Europe's Drugs Bazaar - Portugal
http://www.youtube.com/watch?v=qEDZXuRcGig
