Quote from Milton Friedman,
"[Drug use] does harm a great many other people, but primarily because it's prohibited. There are an enormous number of innocent victims now. You've got the people whose purses are stolen, who are bashed over the head by people trying to get enough money for their next fix. You've got the people killed in the random drug wars. You've got the corruption of the legal establishment. You've got the innocent victims who are taxpayers who have to pay for more and more prisons, and more and more prisoners, and more and more police. You've got the rest of us who don't get decent law enforcement because all the law enforcement officials are busy trying to do the impossible.
And, last, but not least, you've got the people of Colombia and Peru and so on. What business do we have destroying and leading to the killing of thousands of people in Colombia because we cannot enforce our own laws? If we could enforce our laws against drugs, there would be no market for these drugs."