Not gonna try to sell you on the notion that drugs are a net social good. I am going to try to persuade you that having the state dictate what otherwise free citizens may buy and sell, own, or use, or put in their bodies is an outrage against human liberty.
All of the posited objections to the legal use of drugs arise from two sources: 1) the criminal behavior that is an earmark of any black market and 2) the behavioral problems of abusers -- a tinier percentage of the user population than drunk drivers are of the drinking population. Legalization will serve to limit the former and proper attention to the real crimes involved with serve to limit the latter. Otherwise, drug prohibition fits my Laws of Liberalism -- it is 1) founded in ignorance, 2) focused on irrelevance, and 3) engaged in wishful thinking.
Against this, balance the absolute evil that the rationalizations visit on the population -- more so on the innocent than the guilty. Such as Policing for Profit.