It's time to legalize the sale, possession, and private use of marijuana, and not merely for medicinal purposes.
The costs of the current prohibition on marijuana are simply not justified, in any way. Polls indicate that the same proportion of the American population is smoking pot in 2005 as was smoking 30 years ago, prior to the commencement of the War on Drugs in the 1980s.
Despite their inefficacy, programs to discourage the use of pot receive around $4 billion annually from the federal government. In addition, a significant percentage of people in jail are there for charges related to trafficking, possession with intent to sell, repeated possession, and (in some states) a single incident of possession of marijuana. Some federal judges have estimated that they spend 50% of their time in the courtroom dealing with low-level marijuana prosecutions. The criminal justice system is an extremely costly institution for the government to run, and we don't need to be wasting tax dollars on such petty offenses.
Additionally, eliminating the black market for marijuana would undoubtedly curb at least some violent crime. Beatings, robberies, and murders occassionally occur during the course of even minor drug transactions. Legalization of pot would eliminate a certain portion of these "rider" crimes because the contexts in which they would otherwise arise would no longer exist.
Additionally, the government could levy an extremely high tax on marijuana, because it is incredibly cheap to grow and sell. The elimination of the black market would cause the price of marijuana to decrease as there was no longer any risk involved in cultivating, buying, and selling the drug. The government could control the sale of the drug by licensing shops to sell it. The feds could make even more money (in addition to the tax) because it could charge for the licenses needed to sell weed.
Finally, the uncertain effects of marijuana on human health are not a sufficient reason to continue the criminalization of this drug if we are going to keep alcohol and cigarettes legal.
I think that the Democratic party should seriously consider spearheading the efforts to legalize marijuana, as it seems to be the most sensible (and fiscally responsible) approach to the fact that millions of Americans will continue to get high, despite the federal government's best efforts to put out our joints!