This tracks with what CO found after legalization.

I wish it came earlier. Having marijuana be illegal contributed to its place as a gateway drug. Not to mean that using marijuana leads to using other drugs but when you need to buy it from a black market drug dealer, you are more likely to be exposed to people who do other drugs.

In all my years as a heroin addict and dealer I can say not everyone I knew who smoked weed shot heroin, but nearly everyone I knew who shot heroin also smoked weed.