Marijuana use and the California Compassionate Use Act

Congress enacted the Controlled Substance Act, 21 U.S.C. §801 (CSA), as part of the Comprehensive Drug Abuse Prevention and Control Act of 1970. CSA establishes or designates certain drugs as “controlled substances” and makes it unlawful to knowingly or intentionally “manufacture, distribute, dispense, or possess with intent to manufacture, distribute, or dispense, a controlled substance.”

After the 1970 National act, California voters passed a proposition which was codified as the Compassionate Use Act (CaCUA). Among other purposes, the primary purpose of the Act intended “to ensure that patients and their primary caregivers who obtain and use marijuana for medical purposes upon the recommendation of a physician are not subject to criminal prosecution or sanction.” The Act creates an exception to California laws prohibiting the possession or cultivation of marijuana for patients and primary caregivers (as defined by California law) who cultivate and dispense.

What power does the federal government have to regulate the State of California and its residents as it relates to the CaCUA, which seems to directly conflict with the CSA?