As far as the California pot heads exporting to other states, that would come under Interstate commerce, but one could also say the same thing about foreign sources, although in those cases it's often not legal in the country of origin, and that doesn't seem to make any difference in folks exporting it to the US.
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I heard that the clinching arguement was that Dcotors might overprescribe it and therefore create a supply that could be transfered outside a state. Surely this arguement would outlaw every other prescrbed drug?
Personally I think MJ is around the same level as Tobacco and Alcohol in terms of danger tot he user but it has medical benefits. A liquid version has been "medicalised" for use by a UK company and is called Savitex. Apparently it beats Marinol hands down.
As far as fatalities are concerned (DUI deaths and violent crime, etc.), alcohol is probably the worst drug on the face of the earth, causing more untimely deaths than all other drugs combined. Hence the hypocrisy of the Drug War.