Let us not forget that the US Govt. already tried to poison marijuana smokers in the 70s by supplying Paraquat to the Mexican Govt to spray on Pot Plants before it was smuggled into the US and consumed by US citizens.
http://en.wikipedia.org/wiki/Paraqua...araquat_pot.22
The program had little success as a deterrent or to reduce the potency of the plants. 20 years later no large scale or long term issues were reported from smoking pot sprayed with it.
However..............
http://en.wikipedia.org/wiki/Paraquat
http://www.nlm.nih.gov/medlineplus/e...cle/001085.htm
like other herbicides and defoliants used by the US Govt the real danger/threat was to the person(s) applying the chemical to the plants.