A simple way to measure the toxicity of a drug is to compare the ratio between gaining the desired effect (getting high or a buzz) and a lethal dose. Heroin is lethal at five times the effective dose, alcohol is ten times, cocaine is fifteen times. Where does marijuana fall? You would need to consume more than one thousand times the dose needed to get high to lethally overdose. This number is theoretical however, as there has never been a known case of death by marijuana. In comparison, about 24,000 people die every year due to over-drinking.
The classification of marijuana as a dangerous drug is arbitrary and non-scientific. The U.S. Federal government has a duty to create laws that are just. Classifying a lethal drug as legal to sell and consume while also classifying even possession of a non-lethal drug as a criminal offense is neither just nor reasonable.
Gable, R. (2006, May/June). The Toxicity of Recreational Drugs. American Scientist, 94(3). Retrieved from http://www.americanscientist.org/issues/pub/the-toxicity-of-recreational-drugs/1
Centers for Disease Control and Prevention. (2011, December 7). Alcohol Use in the U.S.. Retrieved February 8, 2012, from http://www.cdc.gov/nchs/fastats/alcohol.htm