Depends what criteria you put on "proof", really...
The internet is brimming with statistics of Portuguese drug use plummeting, numbers talking about 60% drop, and the drug-use being the lowest in Europe.
If you want government level access type of documentation, I'm afraid I can't help you there
https://en.wikipedia.org/wiki/Drug_policy_of_Portugal <---widely available data
Observations include
more people welcome rehabilitation, fewer youngsters start with drugs, and fewer overdose deaths. Win, win, winnily-win.
Also, once again, this isn't about some drug-haven free-for-all philosophy.
Police in Portugal can seize someone's drug posession. They just won't throw them in jail.
Portugal has not legalized drugs, the government is not producing drugs. Smuggling is still illegal. I still see full legalization as ideal, because it would eliminate all needs for smuggling.
Why would people want to import impure over-expensive drugs from armed criminals, when they can buy it locally, under government scrutiny? And when I say "they" I mean pharmacies, licenced establishments, nobody's imagining some big "farmers market" with packs of white powder.
It's about taking control of the situation, instead of leaving it to the mobsters, while imprisoning the victims.