https://papers.ssrn.com/sol3/papers.cfm?abstract_id=1464837 It's actually not drug use which has decreased all that much - it's actually "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."

This obviously has nothing to do with the crisis.