Banning substances naturally decreases use, that's obvious, but prohibition criminalizes use, which will always persist. You cannot stop drug use. Drug legalization so far has resulted in declining use of dangerous substances like tobacco and alcohol. Far more young people today choose smoking pot over smoking cigarettes and drinking alcohol. Many people choose not to drink because they've observed the widespread dangerous effects of it since it's been legal. If heroin was made legal all over the world today, you'd 100% see increased use. But maybe humanity needs to see the consequences in order to respect them? Just like alcohol and cigarettes?