The Food and Drug Administration, the Drug Enforcement Administration and the U.S. Public Health Service have rejected smoking crude marijuana as a medicine on account of the following harmful consequences: premature cancer, addiction, coordination, and perception impairment, a number of mental disorders including depression, hostility and increased aggressiveness, general apathy, memory loss, reproductive disabilities and impairment to the nervous system.*
Marijuana was outlawed in 1937, following the Marijuana Tax Act.  Interestingly, "the U.S. Congress passed the act partly in response to many sensational news stories, spread, some say, deliberately and falsely to hasten the illegalization of the drug" (Liska, 50).
"HIV-positive marijuana smokers progress to full-blown AIDS twice as fast as non-smokers and have an increased incidence of bacterial pneumonia."*  
Marijuana contains many of the same carcinogens as tobacco; in fact, the amount of cancer causing agents inhaled from smoking five joints a week can be equal to the amount from smoking one pack of cigarettes a day (Wu, Taskin, Djahed, and Rose, 349).
"Marijuana is currently up to 25 times more potent than it was in the 1960s."*
The Gateway Debate: "While a biomedical or causal relationship between marijuana and the use of hard drugs has not been established, a strong correlation exists.  Twelve to 17 year-olds who smoke marijuana are 85 times more likely to use cocaine than those who do not. Sixty percent of adolescents who use marijuana before age 15 will later use cocaine."*
Photo courtesy of Jean Hanamoto
"The main psychoactive ingredient in marijuana, THC (Tetra Hydrocannabinol), is already legally available in pharmaceutical capsule form by prescription from medical doctors. This drug, Marinol, is less often prescribed because of the potential adverse effects, and there are more effective new medicines currently available."*

    (*Unless otherwise mentioned, this information was taken directly from www.usdoj.gov/dea/pubs/sayit/myths.htm).