"Many believe marijuana is a gateway drug??perhaps not so harmful in itself but one that leads to the use of more serious drugs. That is not borne out in practice, except that the illegal purchase of cannabis often exposes consumers to profit-minded dealers who push the hard stuff. In this way, the gateway argument is one in favor of decriminalization."
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I agree with the doctor, but even here, I think he concedes too much. Having no other way until recently, I engaged in "the illegal purchase of cannabis" for almost 30 years, and never in that time did my dealers deal in harder stuff like cocaine. They all had hash on occasion, but hash is cannabis; and some of them had mushrooms every so often, but mushrooms are closer to cannabis than to cocaine.

We tend to lump everything under the category of "drugs," but just like a gin distiller knows nothing about crushing grapes, drug dealers tend not to be interdisciplinarian.