I believe marijuana should be legalised + regulated, it would be far less dangerous than the huge black market operations. I also think smoking tobacco is disgusting, but in the same vein, shouldn't be banned because then you'd have a horrible black market (and it would never happen anyway, tobacco companies are too powerful).
Also, legalising hard drugs would be beneficial in the long run - once things are out in the open, they can be made much, much safer. Right now, it's easier for under 18s to get hold of illegal drugs because it's no questions asked, whereas with alcohol and cigarettes, there are well-enforced laws and ID checks. The level of underage drug use would fall so much if drugs were legalised, made adults-only, and supplies strictly regulated.
Remember America's alcohol prohibition? That didn't turn out so well...