Meth is definitely poison. It kills. But illegal meth is made in all kinds of insane ways that makes it even more poison; and as the illegal meth trade expanded in Portland, there was a corresponding explosion of Identity Theft related crimes. Perhaps with legal meth and known buyers you gain some degree of control over these problems. Problem is that we associate legalizing a vice with sanctioning it. We need to get over that. Legalize it to control it.

The drug user with the discipline to not use because it is illegal is I suspect rare. I don't not use because it is illegal, I don't use because I understand the longterm consequences are far more devastating than any shortterm benefit. Those that do use will do anything to get their next hit, all sense of morality, let alone criminal deterence, is a distant thought with little deterence value.