If we legalized so powerful/bad drugs like heroin or cocaine we build a business, if we legalize those drugs then we need to legalized newer drugs, if we let corporation to make money out of drugs, well the first scenario that I can think about is a big pharmaceutical corp. who is investing in order to build super addicted drugs, super cheap drugs (free for the new-consumers ???) and commercial like "Try fancy-cool-drug-name you will get smarter, more beautiful and every woman will want you" (and at the very end a little voice will said very very fast "It can give you dependency")
I mean, I already hate the commercial of antidepressant (that are about the same stuff) and I am sure you do the same, legalize everything will be too much.
There is only one single scenario where it would be possible, when we will Educate (note the capital E) every single person, then no one will use such powerful drugs (heroin, cocaine and acid, i don't know anymore).