I'd hate to get all true scotsman but a true leftist would never preach for prohibition as a solution for vice.
That's what happens when you squash a multi-dimensional space of political beliefs down into a single dimension of left-right. You can't have a meaningful discussion about anything from this starting point.
Viewed from a 2-dimensional spectrum this problem lies on the social authoritarian-libertarian axis, not the economic left-right axis.
I'd consider myself a "true leftist" and while I don't think prohibition usually works, I also don't believe in absolutes of liberalism where everything goes - where corner shops can sell heroin and if you fall into addiction that's just your own moral failing.
I support individuals' freedom to use drugs in a controlled, responsible manner, but there need to be limits somewhere to protect naive individuals from getting themselves into something they'll regret and to protect society from collapsing.