the more illegal they are, the more "black market" they become, and thus, there is less telling what's inside them. i sold/traded a little bit of blow and weed in high school like a lot of kids did, and i can say for a fact blow was often cut with all KINDS of different shit. i can only imagine what's in things now.
people who are addicted to drugs are going to do them no matter what the legality is, and for people who aren't addicted, the legality doesn't mean anything. they will use as needed/wanted and the demand is low.
the safest possible outcome is education regarding drug abuse and long-term use effects, rehabilitation funding, and legality/regulation for every single recreational drug there is.