As I have mentioned before, cirrhosis of the liver rates fell roughly in half within a few years of the start of Prohibition. They came back up again (although not as quickly) with a few years of the end of Prohibition. Pretty clearly, alcohol consumption, and especially the regular, high consumption of alcohol associated with cirrhosis of the liver, fell because of Prohibition. Whether the other negative consequences of Prohibition (such as gangsters and corrupt politicians) was too high of a price to pay is a legitimate question. It is also a legitimate question whether Prohibition disproportionately discouraged those drinkers who weren't the social problem. I rather suspect that people that had the occasional beer before Prohibition, or some wine at home with dinner, weren't the ones hitting speakeasies--and they weren't the problem that Prohibition was trying to fix. But let's not pretend that prohibiting a commodity doesn't affect consumption rates.
First of all, I concede that of course prohibition, whether of alcohol (then) or illegal drugs (now) does affect consumption rates. When you raise the price of something, you have less of it -- that's darn near close to a universal truth.
That said, "gangsters and corrupt politicians" weren't the only cost associated with alcohol prohibition, though it is the one that libertarians tend to talk about the most when discussing the parallels between alcohol prohibition and the current War on Drugs. Rather, one of the costs was that people who wanted to drink were prohibited from doing so. Clayton is right that "people that had the occasional beer before Prohibition, or some wine at home with dinner, weren't the ones hitting speakeasies--and they weren't the problem that Prohibition was trying to fix." They may not have been "the problem Prohibition was trying to fix" -- although Temperance crusaders, like drug warriors, weren't big on the distinction between recreational and abusive uses. But so what? It was illegal for them, too. And because they were, for the most part, law-abiding people who enjoyed some beer or wine but were perfectly willing to do without, they generally did without rather than incurring the risks of breaking the law. And they were thereby deprived of the experience of having a bottle of wine among friends over dinner. Taking that pleasure away from people -- well it's obscene.
I am fully aware that alcohol has very real social costs. And yes, I think that Al Capone and the other gangsters and hoodlums who prospered during Prohibition were indeed a good reason to do away with it. But the infringement on liberty alone was sufficient reason to object to prohibition. I enjoy having a glass of wine with dinner from time to time, or a beer with my pizza, and I don't see why I should be deprived of that pleasure because some people cannot drink responsibly.
Likewise, the War on Drugs has very real costs, some of which I've touched on before. I think those costs are good and sufficient reasons to get rid of the war on drugs. But that's not the only reason. For the most part, I think I think that taking now-illicit drugs is a bad choice, and if the sorts of drugs that people now use recreationally were legalized, I doubt I'd partake in them. But that would be my choice. People who decide to use heroin or meth might well be throwing their life away, making a very poor choice. But it's their life, and their choice. They should have the freedom to make it.