17 states + DC. 17 states and washington dc have passed medical marijuana laws, but they vary in how they're enacting them. Each state may also have different processes. I don't at all recommend that people expose themselves to the medical and psychiatric dangers that using marijuana poses. Still, this web site addresses your question: http://medicalmarijuana.Procon.Org/view.Resource.Php?Resourceid=000881.
Pot. Physicians do not support any use of pot. It makes treatment of mental disease impossible. It triggers early appearance of severe psychosis on people that would have developed milder ones and much later in life. Only criminal doctors support any form of drug use. Healthtap would not condone any doctor replying to your question.