Did you know that since 2001 heroin, cocaine, marijuana and all similar drugs were decriminalized for personal use in Portugal?  Decriminalization means that there are no criminal penalties, and instead citations for use result in the offer of counseling and voluntary rehabilitation.  This CATO study reports that since then use of these drugs has actually declined.

The study is interesting for the historical account and other context but the claim about usage rates is hard to believe.  The paper is missing details on how usage rates are calculated but a simple alternative explanation is this.  Without criminal penalties, law enforcement has reduced incentives to issue citations and thus users are undercounted.