Laws exist to protect individuals from the acts of others; not to prohibit self-abuse (in any degree). End the control of any substance that isn’t capable of directly killing a lot of people (i.e. chem warfare stuff). If someone wants to commit suicide (verrrrrrrry slowly in some cases), that’s their problem, not the government’s.

As for the gateway drug argument. Evidence to the contrary notwithstanding, so what? Since when is it the business of the government what a person does to him or herself?