(Well, not really an experiment, as they’ve evidently decided that it was a good idea and look to keep it going.) Perhaps oldish news, but I came across this this last month and found the data interesting. An analysis of the effects of Portugal’s 2001 decriminalization of all personal drug use says:
“…decriminalization has had no adverse effect on drug usage rates in Portugal, which, in numerous categories, are now among the lowest in the EU, particularly when compared with states with stringent criminalization regimes. Although postdecriminalization usage rates have remained roughly the same or even decreased slightly when compared with other EU states, drug-related pathologies — such as sexually transmitted diseases and deaths due to drug usage — have decreased dramatically. Drug policy experts attribute those positive trends to the enhanced ability of the Portuguese government to offer treatment programs to its citizens — enhancements made possible, for numerous reasons, by decriminalization.”
The paper on that site is an interesting read. Portugal’s system appears to have its weaknesses, but it is an example of what has (and hasn’t) happened what a society switches its focus from throwing drug users in jail to attempting to treat them instead.