I thought of this idea for the DEA a long time ago. It would be tough to get the drugs into the coke between the cartels and the dealers without someone knowing what is up. If it could be pulled off, then a lot of people dying would probably put a serious damper on drug use. It would be pretty much impossible with locally produced drugs like meth and pot though.
No kidding, if the DEA could get magiaclly get their hands on the drugs to poison it, why not simply destroy it? Because killing people is more fun?