Some common arguments, I won't elaborate on any but it will give you ideas.
1. If you include all the police expenses, jail costs, propaganda against marijuana, etc., fighting cannabis costs the United States government $42 billion every year. Think of all the things that amount of money could go to, like... I donno, fixing the economy. Education. Fighting real drugs.
2. On top of the money the government would save, it would also earn a truckload by selling taxed marijuana, similar to cigarettes. This has the added benefit of taking money away from criminals (drug dealers) and putting it into the hands of the government.
3. The number one way to keep kids away from marijuana is to legalize it. Ironic, but true. How much harder is it for the average high school student to get alcohol, or even cigarettes, than a sack? Much, MUCH harder. When you force a substance underground, anyone can sell it, and dealers are easy to find. If you have to find someone of age it becomes much more difficult and annoying.
4. Much more dangerous and destructive drugs, for example Oxycontin and Vicodin, both powerful opiates, are sold legally every day over the counter. Marijuana, even though it is far less harmful, remains illegal.
5. Alcohol is much more damaging than marijuana. It doesn't impair coordination/judgment nearly as much, it doesn't cause blackout rages, etc. It isn't chemically or physically addictive. It doesn't trash vital organs to nearly the same extent. It doesn't cause deformed fetuses. It's just better for you, and more natural. In fact, our brains have NATURALLY OCCURRING cannabis receptors.
6. Marijauna has been proven numerous times, by extremely credible institutions, to actually fight and prevent against several devastating cancers. These studies were promptly silenced by the DEA, but you can find them online.
7. The penalties for the possession of marijuana are much more harmful to the user than to the actual drug itself. Fines, court dates, occasional jail time, the negative stigma associated with being a "drug user", the difficulty of getting a job or loan when every application requires you to write down that you got busted with pot... All it does is make you happy, appreciative of the world around you, and less stressed. Which sounds worse?
Other Interesting Shit
Hemp is a much more durable, environmentally safe, comfortable, quality product than cotton. The cotton industry obviously didn't want to go out of business, so they asserted superiority by associating hemp with risky and dangerous drug use.
The reason it is called 'marijuana' and not 'cannabis' is because lawmakers wanted voters to associate weed with illegal immigration, something many people felt very strongly negative about. So they promoted the Mexican name for cannabis, marijuana.
Many of the "facts" you see in anti-drug propaganda are taken from research done by agencies paid by the DEA or other government agencies to produce exactly the results they "found".
Good luck, dude.
