Okay, here’s the basic idea. We don’t need to prohibit drugs because harmful drugs have a built-in deterrent. A vast majority of people won’t use these substances regardless of their legal status because they are deterred by the built-in risks. Legal penalties obviously don't hit the people who don’t use, but are gratuitous and cruel to those individuals who use anyway and get caught. Also, many substances are straight-up benign, despite their nasty reputations. The voting public and their government representatives are very bad at discriminating dangerous from benign substances. We know that the demand for mind-altering drugs is low (at any rate saturated) because some are readily and legally available and almost nobody seems interested in using them. If drug prohibition has little or no deterrent effect, the jig is up. Legalization won’t lead to a significant increase in usage, so there is no reason not to.
Assuming you do want to deter drug use anyway, there are far more sensible ways to do it that don’t create a black market and that don’t initiate violent confrontations between law enforcement and “criminals.” A tax on drugs and user-licensing can provide a general deterrence, subsidies for treatment can help vulnerable individuals, and if necessary restrictions on advertisement and age restrictions can actually be implemented in a legal regime. Some of these options are difficult or impossible under a prohibition regime, but are obvious and easy to implement under legalization.
The most important reason to legalize drugs is the moral reason. Fundamentally, people own their bodies. The government does not own you or the contents of your brain or blood-stream and has no right to dictate their contents. The point here isn’t to play an all-powerful moral trump card, just to state where the presumption lies. You can overcome a moral presumption, even a very strong one. But you’d better have a damn good reason for it. Do you have one in this case?
It might take me slightly longer than an average elevator ride to recite all this. I think that’s okay. I could instead make the pitch be any one or any two of the above three paragraphs. As the links imply, I can defend any of these statements in much greater detail. That is my preference: to anticipate objections and respond to them. That’s how my mind works. I think it's a healthy habit. But it makes concision difficult. That is my great frustration. I feel like I can fully justify this argument, but to do so takes a much longer article or lecture than any skeptic is willing to read or hear. You may read the above as three separate attempts at concision.