The new policy is in line with what candidate Barack Obama promised in his campaign. As a presidential candidate, Obama said he would end federal raids on people who used marijuana for medical purposes.

"The Justice Department going after sick individuals using this as a palliative instead of going after serious criminals makes no sense," Obama said in July 2007.

The new formal guidelines reflect that language. "As a general matter, pursuit of these priorities should not focus federal resources in your States on individuals whose actions are in clear and unambiguous compliance with existing state laws providing for the medical use of marijuana," states the memo from the Justice Department to U.S. attorneys.

"For example, prosecution of individuals with cancer or other serious illnesses who use marijuana as part of a recommended treatment regimen consistent with applicable state law, or those caregivers in clear and unambiguous compliance with existing state law who provide such individuals with marijuana, is unlikely to be an efficient use of limited federal resources," the memo states.