Well, let's see. It's been clinically proven that breathing oxygen can cause a buildup of free radicals in your bloodstream that will eventually kill you, so we'd better ban air too, just to be safe.
Okay, I'm being facetious here, but honestly. We already tried banning stuff like alcohol, and it didn't really work. A better idea might be to make its use and abuse as legal, and VISIBLE, as possible. I think if we all saw a share of meth-addicts with corroded teeth and coke addicts going through withdrawal on the street corners, less of us would be inclined to try drugs of any kind.
That, and I'd wager a lot of drug use starts out as rebellion. Somehow, I don't think saying, "hey guys, let's go take some of this perfectly legal substance that will severely damage our health and won't get us in trouble in any way" has quite the same advertising appeal.