You know, the Libertarian positions on Drug Prohibition and immigration were the two issues that stymied me for a while; so I fully understand the hesitation some have with the libertarian position on these issues.
Over time, I realized that Drug Prohibition has been the single largest cause of the loss of liberty in general and the most serious factor undermining the protections enshrined in the Bill of Rights specifically. Alcohol Prohibition had the same effect while it was in force, too.
Drug Prohibition is unconstitutional; despite the inability of the Supreme Court to say so. We can't help it if law graduates don't know how to read. In any case, there was never any Amendment authorizing the federal government to regulate, much less prohibit, the recreational use of any substance.
Libertarians understand that there are costs to the individual, and to society, of drug use. No rational Libertarian advocates the abuse of any drug. That said, we believe Drug Prohibition is a "cure" that is far worse than the disease.
One of the many concerns about our position on Drug Prohibition, is that if Prohibition is repealed, there would be a massive spike in the number of users. I tend to agree, but my personal theory is that this effect will most likely be due to congressmen, state legislators, and cops all heading en masse to their local drug store -- with teenage pages and interns in tow -- as soon as Prohibition officially ends. Don't expect any of these people to show up for work the first couple of weeks after Repeal.
In all seriousness, if you examine the trend in prohibited markets, it is always towards harder, more potent varieties of whatever the substance is. The history of Alcohol Prohibition showed this phenom very clearly: as Prohibition started, traffickers brought in anything people wanted. Over time, however, they began to gravitate towards ever-more potent hooch and other hard liquors; and of course always there was the danger of adulterated product.
After Repeal, years of persistent education pointing out the dangers of too much alcohol has led to wine coolers, non-alcoholic beers, and otherwise less and less potent varieties of social beverages.
Likewise, with Drug Prohibition, the trend has been to ever-more potent varieties of drugs, a constant search for better "delivery devices", and again adulterated product. With repeal, besides bringing street disputes (which currently end in bloodbaths) into the court system for resolution, the trend will be towards unadulterated and increasingly safe products.
In addition, keep in mind that Repeal advocates do not suggest that drug users should be permitted to operate motor vehicles irresponsibly. Everyone, even drug users in an environment without Prohibition, should still be held 100% responsible for their actions.
The side effect of Prohibition that does the most damage is that trade in illegal drugs finances terror around the world. The enemies of the United States routinely use drug trafficking to finance operations against US soldiers, civilians, and other targets (look at the resurgence of the Taliban in Afghanistan, not to mention the narcodollars propping up communist thieves Morales & Chavez in South America. We absolutely *must* remove this funding source from these crooks' toolkit. Continuing to ratchet up the drug war will do the opposite -- it will keep them in business.