Actually, all available evidence points to the fact that legalising drugs like heroin actually decreases their usage. Would you suddenly take up heroin if it was legal? Do you know anyone who would?
If marijuana were legal I might use it. Conversely, I don't think I know anyone who uses marijuana/heroin but would stop using it if it were legal.
I haven't seen the evidence either way; I just made the point in my original post as something to consider.