Prohibition has been unsuccessful many times in history, just look at what happened during alcohol prohibition. It causes crime to go up because of the fact that it is illegal. The black market profits go way up because you can't legally obtain it, which causes violent crime to increase because of the existence of the black market. Prohibiting drugs also makes it "forbidden fruit" and taboo, which is a big reason a high number of teenagers and young adults use drugs.

Doing drugs is a victimless crime and it should be a person's choice what they put in their own body, not the government's choice.