@ Holk: Maybe, but consider the following.
Maybe the hardcore junkies were starting with pot to see if it would satisfy their "needs", and when they found it did not, they tried other things. Perhaps even several things. I would disagree with pot somehow leading to other garbage in and of itself. I think people who end up using harder drugs are simply in an environment where illicit substance use is accepted, and thus have "connections" to dealers who can supply their wants AND a social structure which condones such behavior, ultimately leading to validation of their desire to use other substances. In other words, the opportunity , the desire and some kind of social or emotional enablement must be present.
That's what I think.