"Reprinted with permission fromMARIJUANA LEGALIZATION: What Everyone Needs to Know by Jonathan P. Caulkins, Angela Hawken, Beau Kilmer, Mark A. R. Kleiman, published by Oxford University Press © 2012 Oxford University Press. 
If alcohol is more dangerous than marijuana, what’s the logical justification for one being legal and the other illegal?
If we were making laws for a planet whose population had never experienced either marijuana or alcohol, and we had to choose one of the two drugs to make available, there would be a strong case for choosing marijuana, which has lower organic toxicity, lower addictive risk, and a much weaker link with accidents and violence.
But that’s not the planet we inhabit. Here on this planet, alcohol has been an ingrained part of many cultures since the Neolithic revolution (which may have been driven in part by the discovery that grain could be brewed into beer). People have used cannabis plant products for thousands of years, but its widespread use as an intoxicant in the United States is a phenomenon of the last hundred years. Even today only about one in sixteen American adults used marijuana at all in the course of a typical year; for alcohol, that figure is more than half.
History matters. Custom matters. Practicality matters. Even if there were public support for it, going back to Prohibition wouldn’t work—without a truly ferocious degree of law enforcement—precisely because centuries of tradition and decades of marketing have left alcohol use a deeply ingrained feature of most social systems outside the Islamic world.

The technical term for this is “path dependence.” If alcohol had just been invented and no one was yet using it, it would go straight into Schedule I: high potential for abuse, and no accepted medical value. And that ban might make sense. But once there is an established user base, prohibition becomes impractical. Marijuana is not, or at least not yet, equally entrenched.

It’s true that the arguments for maintaining marijuana prohibition also point strongly toward tighter controls on alcohol: higher taxes, limits on marketing, bans on sales to people convicted of driving drunk or other crimes committed under the influence. So it’s fair to mock the “drug warriors” who worry about every drug except the one that does the most damage. But, by the same token, advocates of marijuana legalization who point to the horrible amount of damage alcohol does are pointing to precisely the strongest argument for maintaining marijuana prohibition: the one drug we legalized is the most harmful of all despite all our efforts at regulation."