Introduce poison into drugs to kill off dregs of society who are already poisoning themselves?

I recently heard of, but have not yet found or read, of two men who submitted their doctoral dissertation suggesting that the government introduce poison into the drugs that are being sold on the street (coke, meth, heroine etc) arguing that they are already poisoning themselves, this would simply do it faster. The result would be a quick elimination of the dregs of society. I live in an area with a lot of tweakers (meth addicts) and I have found myself growing fond of this idea. I was wondering what people thought about this concept; ethical concerns? better solutions etc...