I'm going to copy/paste one of my earlier arguments because I feel that it properly argues for legalization from many angles. And on the other debate no one argued against me, so I welcome anyone to disagree if you can actually make an argument other than "Well it's illegal therefore it's bad therefore it should be illegal."

I will argue first from a practical position, then philosophical, then historical.

Marijuana is not dangerous. There has never been a death recorded due to marijuana. The LD50 or amount that it would take to kill half the population is incredibly high. The estimate is 1500 pounds of marijuana in 14 minutes. You can reasonably say it is so high, it is impossible to die from overdosing on marijuana. Also marijuana does not make you stupid. The reason people think this, is that there was a study done on monkey where they let them breathe only marijuana smoke and they died from hypoxia or lack of oxygen. When you die from a lack of oxygen, your brain cells die. The severe errors in the study, however, were released much later than the results so there is now a stigma that marijuana use makes you stupid. They have found that smoking marijuana does not cause cancer like cigarette smoke. A UCLA study found no link. The gateway drug theory is actually perpetuated by the prohibition of marijuana. Because marijuana users must get pot from drug dealers, they are exposed to harder, dangerous drugs. If marijuana was able to be sold in stores, this would not happen. Legalizing marijuana would not lead to an increase in use by children. According to many studies, it is vastly easier for teenagers to get marijuana than it is for them them to get alcohol. This is because alcohol must be purchased through legal channels where vendors check for IDs. However, because of the black market for marijuana drug dealers do not care how old someone is when they sell them pot. The legalization of marijuana will be very good for the economy. We spend billions of dollars each year to keep people in prison because they like to get high and relax. Additionally marijuana is a huge cash crop the government could make a lot of money off from taxation. It is the number one cash crop in California for example. ~60% of the money of Mexican drug cartels is earned by selling marijuana in the US. Severely compromising their funding would reduce crime in the United States as well as Mexico, which would help improve Mexico's living conditions and curb illegal immigration into the United States.

The government should not have a right to prohibit you from doing something to yourself that does not harm anyone else. It is not illegal to go home and drink a gallon of paint thinner, but I guarantee that it's much better for both you and society that you smoke marijuana instead of drinking paint thinner. Here's a more reasonable example: Should the government make it illegal to consume significantly more calories than you expend? This is much more harmful than marijuana, and obesity related illnesses are one of the leading causes of death. The bottom line is that this type of reasoning that the government needs to be able to protect you from yourself is dangerously close to fascism. Where do you draw the line? Do we monitor people's jobs and make it illegal to not work hard to make sure people are productive and that they get promoted? Do we make it illegal to be late for job interviews because that is bad for you? Do we make it illegal to sleep in beds instead of on the floor because people have died this way and no one has ever died from marijuana? The answer is no. Adults are free to make their own decisions that only affect themselves. We do not live in a "nanny-state" as Ron Paul puts it.

Many people assume that because marijuana is illegal serious research and thinking went into the decision to ban it. This simply isn't true. The first marijuana laws in the United States forced people to grow marijuana during colonial times. Marijuana became illegal for a few reasons. One was due to extreme anti-Latino and anti-black sentiment. People were fear mongered into believing that minorities were a danger to white society especially due to marijuana. The "reefer madness" campaign created undue hysteria. Yellow journalism and personal career advancement also played a role. In fact President Nixon commissioned a report on marijuana which concluded that marijuana should not be illegal. http://www.druglibrary.org/Schaffer/Library/studies/nc/ncmenu.htm

The next logical question is why is marijuana still illegal. Again there are a few reasons. What kept it illegal during the middle and late 1900s was again a fear mongering campaign linking marijuana use and communism. By this point in history there is so much social stigma against marijuana, that if you ask someone against legalization why it should be illegal, they will most likely cite things that simply aren't true. Additionally the alcohol and tobacco industries are absolutely huge. Companies in those industries make a so much money that they can spend vast amounts lobbying congress to keep marijuana illegal. They don't want competition. Why would you drink or smoke tobacco when they are incredibly dangerous and addictive when you can smoke marijuana which is harmless and not addictive? Many people underestimate the effect that a large lobby can have on national and state policy. The alcohol and tobacco companies also perpetuate the social stigma against marijuana by funding anti-drug and anti-marijuana advertisements and programs. The goal of these programs is not to make you not do drugs, its to stop you from doing drugs that they don't sell you. Also marijuana for medicinal purposes is a huge threat to the pharmaceutical industry. The pharmaceutical industry is the biggest industry in the United States. Just like the tobacco and alcohol companies, pharmaceutical companies spend millions of dollars lobbying congress. Marijuana has been said by some doctors to be the single best drug for maintaining health. It combats an astonishing number of diseases and conditions, and has no side effects. Compare that to the multitude of side effects from prescription drugs. It has become a cliche how at the end of television commercials for prescription drugs they rattle off a long list of some pretty serious side effects. Thousands of people die from adverse reactions to prescription drugs.

As I hope you can see, the prohibition of marijuana is completely wrong on many levels. Making marijuana legal will keep it away from kids, keep people away from dangerous hard drugs, and allow adults to make their own choices when it comes to their own lives.