This is what evidence-based policy looks like

Following decriminalization, Portugal eventually found itself with the lowest rates of marijuana usage in people over 15 in the EU: about 10%. Compare this to the 40% of people over 12 who regularly smoke pot in the U.S., a country with some of the most punitive drugs laws in the developed world. Drug use of all kinds has declined in Portugal: Lifetime use among seventh to ninth graders fell from 14.01% to 10.6%. Lifetime heroin use among 16-18 year olds fell from 2.5% to 1.8%. And what about those horrific HIV infection rates that prompted the move in the first place? HIV infection rates among drug users fell by an incredible 17%, while drug related deaths were reduced by more than half. "There is no doubt that the phenomenon of addiction is in decline in Portugal," said Joao Goulao, President of the Institute of Drugs and Drugs Addiction, at a press conference to mark the 10th anniversary of the law. 

But to quote another famous philosopher:

What good are facts? With facts, you can prove anything that's even remotely true! - Homer Simpson

The point of the drug war, of course, is not to prevent drug use. It is to punish a certain class of people and marginalize them from civic life (and of course the political process). And how do we know this? Because if the goal of policy were to actually contain drug use, it would - like Portugal - be concerned with actual numbers.