I've seen a microcosm to what you're proposing- I've seen the sadness and anger left in the wake of drunk driving fatalities, or someone mixing alcohol and firearms, or the wreckage of someone having one too many tokes before taking his kids with him  to a liquor store before going home.
This part I'm going to have to disagree with, we all know, that making something illegal does not stop those who want to indulge from indulging.
It would be the same as banning guns, the criminals and those who refuse to buckle under and turn in their guns, would still have guns.
People who do drugs are going to do drugs, obviously, or we wouldn't have a drug problem. People who don't do drugs aren't suddenly going to jump up and start buying drugs just because it's  legal.
If the drugs are regulated like alcohol, there shouldn't be any more problems than there is with booze.
There are always going to be irresponsible people, no matter what the law is.
JMO