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You educate instead of criminalising it.
Cigarettes are perhaps one of the worst drugs out there, every cigarette you smoke takes an average of 11 minutes off your life. If you smoke 50 a day, that's a day off your life every 2 and a bit days. Now why the fuck is that better than taking MDMA?
Alcohol is also much worse than most illegal drugs.
Heck, only several other drugs are more addictive than it.
People aren't going to suddenly start injecting heroin overnight.
Ten Years After Decriminalization, Drug Abuse Down by Half in Portugal - Forbes
That case study from Portugal is pretty compelling. Richard Branson is leading a campaign to get drugs decriminalised in the UK.
However, if drugs were decriminalised who would control the supply? (it would for sure need to be controlled in some way). The government? Private business? How would you regulate that?