An Economic Perspective
I've been over this with several Economists and Philosophers, and I thought some interesting facts might illuminate the discussion. I don't use drugs of any kind, but I am a Libertarian who thinks government's job is to protect us from each other, not ourselves. Cigarettes and alcohol are legal, but I don't smoke and rarely drink, though I believe in the RIGHT to do such things.
#1 Most drug murders aren't the results of junkies seeking cash or drugs, they are between dealers who have double crossed one another in transactions. If drugs were legalized, drug dealers who have grievances against a supplier/customer could sue in court for breach of contract like other businesses. This is why bartenders don't shoot alcoholics (i.e alcohol junkies) who fail to pay their tab: they can just go to legitimate collection agencies and the courts. These legitimate, non violent recourses are currently unavailable to dealers, who must resort to threats of force to keep the people they do business with honest.
#2 Economists have calculated that the amount of money it costs to keep a dealer or user of illicit drugs in prison is 8 TIMES the amount it costs to put a junkie in rehab. Even if you factor in the low percentage of rehabilitation in treatment programs (25%), it is still half the cost to legalize drugs and put all ABUSERS in treatment as many times as it takes. (There is a difference between Users and Abusers of a drug; contrary to what most people have been told not all drug users become addicted to their drug of choice, research shows that many "Hard" drugs are in fact less chemically addictive than cigarettes and alcohol, and we know people who often quit these substances or use them in moderation their whole life with no ill social effect).
#3 I have only heard this from one source, but I have never seen any evidence to dispute it, and the source had a PHD and taught at a major state University: Many of the nastier side effects we see in users of hard drugs like cocaine and heroine, are in fact not caused by the active ingredient, but by the impurities in it because it is made in non-laboratory conditions. For example pharmaceutical grade heroin doesn't fry your brain, and might not even be possible to Overdose on. Presumably, if recreational drugs were legal and regulated by the FDA like our medicines and our food are, they could be produced profitably without the terrible side effects of street drugs.
#4 Most disturbing, economists have calculated that the more successful you are in keeping drugs off the street, and dealers in jail, the more the price rises, and since demand is fairly constant it provides a MASSIVE financial incentive to enter the drug business. Consequently, because more risk is involved and the amount of money greater, drug related street violence increases as well. Many Libertarians and free-market economists now consider this the primary reason we haven't won the drug war: it mathematically can't be done without eliminating the human psychological desire for pleasure(users) and profit(suppliers).
#5 Several studies have popped up finding people who abuse alcohol actually do more damage to their bodies, their minds, and their families than do hardcore drug junkies(We can all imagine having an angry abusive drunk for a father, but its hard to picture an angry, abusive stoner).
This isn't actually an argument in favor of drugs, its simply pointing out that from a social and a health perspective it makes no sense for alcohol to be legal and drugs, not. However, after this country's experiment with Prohibition we know what happens when you outlaw alcohol: people can still get it and it made the Mafia extremely wealthy and powerful (Historians now admit that gangsters basically ran some of America's largest cities thanks to the influence that alcohol profits bought them). The chilling parallel is that today, drugs are so profitable that many drug dealers are better armed (and informed) than the police sent to control them, indeed in some countries the Drug Armies are better equipped than the REAL ARMY.