I'm in no way implying you're making the slippery slope argument, but it seems to me there's a common fear-based subtext to this question in the America media. So, I'm going to try to address what I see as the slippery slope subtext.

I don't think there's a slippery sloper from marijuana to cocaine, especially if marijuana is legalized. The slippery slope argument is like arguing there's a slippery slope from beer to cocaine -- addictive personalities aren't that simple. Further, any evidence implying a slippery slope from marijuana to harder drugs becomes less relevant after legalization, since marijuana users will then have fewer interactions with criminals and less exposure to harder drugs.