Duckling, you ended up not double posting! But if you did it wouldn't be a problem since you were giving 2 lengthy replies to 2 different people.
Going point by point.
1. I meant the figure physical arm vs dependence. According to that figure, cannabis causes both less physical harm and less dependency than tobacco and alcohol.
And remember that smoking marijuana isn't the only way of consuming it. So, the cancer comes from smoking something (be it tobacco or marijuana).
2. A treaty is an agreement between countries. In this case, based on medical facts and politics (as any treaty). So, the substances included on that treaty can be changed by the WHO (again based on medical facts and politics). Any country can lobby for one substance to be included or removed.
And regarding my points. It's true that the overall consumption is reduced when prohibition is in place. However, high consumptions exist with or without prohibition.
See the wikipedia on the US Prohinition. And the thing is: the dependency is what causes the most health problems, and prohibiting will not avoid it. Furthermore, prohibition brings a new problem: increased crime. And you can't avoid this one: if it's forbidden, the price is high; and if the price is high, it's more worthy to take the risk.
2. If your point was about recreational drugs, then both alcohol and tobacco should have the same treatment. Anyway, the taxes can be justified by the costs associated to guaranteeing the quality of the substance and to the treatment of the health problems associated with consumption. I mentioned the revenue as a less significant benefit.
Furthermore, by taxing marijuana, the ones who pay are the ones who can be a burden in the future to the state. Unlike now, that all of us pay taxes and a part of it is spent in the fight against drug dealers.
3. I didn't explain myself very well (I wasn't talking about obesity at all). I meant that drug sold on the streets has a large fluctuations in the quality. And the dealers add other products to the drug just to increase the weight of the package sold. The fluctuations in the quality and the products added increase the risk of health problems.
4. I disagree. If the sentences aren't proportional to the offence (or the risk to the society) then the laws will transmit the wrong impression. Uninformed people may think that substances with similar sentences are more or less the same thing and try any of those.
And forget getting rid of marijuana or any other drug, by the way.
"Evidence of the inhalation of cannabis smoke can be found in the 3rd millennium B.C" It's around for 5000 years, so I do think it's ingrained in the human culture.
So, I still think that consumers will be able to have a better health if marijuana is legalized. It doesn't mean that it could be sold in any store. One way of doing it was selling (or even offering in some specific cases) marijuana in specific places. The quality would be controlled and the side effects too. And the crime related to the illegal commerce of marijuana would drop.