More instructive is to ask “why make something illegal that people use in private,” to which one might reasonably counter “because there are externalities in the form of societal costs.”

This, then, gives us a much more interesting question of “how can we reduce the societal costs of people using substances in unhealthy ways?” Indeed, banning a plant has proven not to be an effective way of accomplishing this.

So what is? I don’t know the right answer for the US, but there are examples of other more successful approaches. In particular, it’s worth looking at Portugal’s decision to treat substance abuse as a public health problem.