Heroin would be legal by your definition. Once somebody has used heroin more than once or twice, they are in no state to judge whether the stuff is bad or not.
If someone wants to kill themselves, the government shouldn't be taking money from you and me to try and prevent the inevitable. As you can see, prohibition isn't stopping anybody from doing heroin - it's actually encouraging heroin use, what with free government-provided needles and everything.
So? Heroin is much more benign than meth. Also, heroin is not God's Gift. It is manmade, God's Gift refined. This gift in its morphine form is true compassion for dying people.