"Although Raich established Congress’s constitutional authority to enact the existing federal prohibition on marijuana, principles of federalism prevent the federal government from mandating that the states support or participate in enforcing the federal law. While state resources may be helpful in combating the illegal use of marijuana, Congress’s ability to compel the states to enact similar criminal prohibitions, to repeal medical marijuana exemptions, or to direct state police officers to enforce the federal law remains limited. The Tenth Amendment likely prevents such an intrusion into state sovereignty."

Source

Garvey, Todd, "Medical Marijuana: The Supremacy Clause, Federalism, and the Interplay Between State and Federal Laws," Congressional Research Service (Washington, DC: Library of Congress, March 6, 2012), p. 5.
http://www.fas.org/sgp/crs/mi…