"The federal Controlled Substances Act (CSA) outlaws the possession, cultivation, and distribution of marijuana except for authorized research. More than 20 states have regulatory schemes that allow possession, cultivation, and distribution of marijuana for medicinal purposes. Four have revenue regimes that allow possession, cultivation, and sale generally. The U.S. Constitution's Supremacy Clause preempts any state law that conflicts with federal law. Although there is some division, the majority of state courts have concluded that the federal-state marijuana law conflict does not require preemption of state medical marijuana laws. The legal consequences of a CSA violation, however, remain in place. Nevertheless, current federal criminal enforcement guidelines counsel confining investigations and prosecutions to the most egregious affront to federal interests. Legal and ethical considerations limit the extent to which an attorney may advise and assist a client intent on participating in his or her state's medical or recreational marijuana system. Bar associations differ on the precise boundaries of those limitations. State medical marijuana laws grant registered patients, their doctors, and providers immunity from the consequences of state law. The Washington, Colorado, Oregon, and Alaska retail marijuana regimes authorize the commercial exploitation of the marijuana market in small taxable doses."
CRS Report for Congress, R43435
Federation of American Scientists: http://www.fas.org/sgp/crs/index.html