It's a bit complex, but I would say a few things:
1) You can't legislate morality effectively.
2) Takes that MJ is 100% healthy or has never led to someone dying simply aren't true. It may be incredibly difficult to OD and die, but a lot of associated behavior and some potential health risks with the delivery method(s) still exist.
3) Honestly, I would be a huge proponent of tying healthcare costs more directly to individual behavior. Legalize MJ, keep alcohol legal, charge health insurance according to the risks you take with your health. The necessary metrics would never be agreed upon, nor would we likely tolerate the violation of privacy rights. We could at least handle it like car insurance; pretty inexpensive until you start racking up tickets and wrecks. Unfortunately, we see cheap healthcare insurance as some inalienable right. I went with no healthcare for quite a while early in life until I could afford it and never once felt disenfranchised or like it was owed to me.
Long story short. Complex problem = no easy answers. Please don't tell the media or politicians, they'll be severely disappointed!