From Drug War Rant [via Avedon Carol at Sideshow]:

The recent study comparing marijuana use in San Francisco and Amsterdam is a real blow to the drug warriors. Not only did it show that prohibition does not reduce marijuana use, there were a couple of other very interesting things in the study. The full report is now available online as text and as a pdf with graphs.

....Note that in the city where prohibition is the rule, there is significantly higher rates of experimentation with other drugs. As opposed to the gateway theory, this indicates that prohibition actually increases the likelihood of using other illicit drugs. As the study notes: The “separation of markets,” in which lawfully regulated cannabis distribution reduces the likelihood that people seeking cannabis will be drawn into deviant subcultures where “hard drugs” also are sold is one public health objective of Dutch decriminalization. Looks like their idea works better than ours.

Avedon adds: "And, as even Harry Anslinger admitted, it wasn't a "gateway drug" until it was made illegal."