For those libertarians who believe that marijuana should be legalized, along with other drugs, consider this:
Being a libertarian requires personal responsibility in all phases of life. If one chooses to use marijuana, heroin, cocaine, meth, crack, etc., that's all right. However, if one chooses that route, then one can't expect society to pick up the dregs and foot the bill for medical care, rehabilitation, etc. One doesn't get it both ways.
Living in society is a series of compromises of personal freedom and liberty. One goes along to get along. If the mores of society call for keeping certain drugs illegal, then so be it.
The alcohol and Prohibition are another argument entirely. People have imbibed alcoholic beverages for thousands of years, and the vast majority have done so without becoming alcoholics. Thousands of times daily, people have the one or two drinks socially, with a meal, etc., without ill effects. Prohibition was social engineering of the worst sort. Alcoholism and excessive drinking was a problem to the Women's Christian Temperance Union, among others, and no one else. The ensuing amendment to the U.S. Constitiution was bad governance and societal over-reaching at its worst.