As part of a larger scheme to regulate drugs and other controlled substances, federal law prohibits the cultivation, distribution, and possession of marijuana. No exception is made for marijuana used in the course of a recommended medical treatment. Indeed, by categorizing marijuana as a Schedule I drug under the Controlled Substances Act (CSA), the federal government has concluded that marijuana has "no currently accepted medical use in treatment in the United States." Yet 18 states and the District of Columbia have decriminalized medical marijuana by enacting exceptions to their drug laws that permit individuals to grow, possess, or use marijuana for medicinal purposes. In contrast to the complete federal prohibition, these 19 jurisdictions see medicinal value in marijuana and permit the drug's use under certain circumstances. Such inconsistencies in federal and state law would generally evoke the constitutional principle of preemption—potentially resulting in a conclusion that because the states permit conduct that the federal government has expressly prohibited, such laws are void as in conflict with the "supreme law of the land." This, however, has not been the case. State laws that exempt from state criminal sanctions the cultivation, distribution, or possession of marijuana for medical purposes have generally not been preempted by federal law. This unique interplay between state and federal law has led to a seemingly incongruous situation in which both the federal criminal prohibition on marijuana and state medical marijuana exemptions coexist. Accordingly, a resident of California who uses marijuana for medical purposes in compliance with California law is nonetheless simultaneously in violation of federal law and potentially subject to prosecution by federal authorities. Such prosecutions, however, are relatively rare. The federal government has limited resources to draw upon in investigating and enforcing federal drug laws. As a consequence, the Obama Administration has formally suggested that it will not prosecute individuals who use medicinal marijuana in a manner consistent with state laws. The legal anomaly that defines the use of medical marijuana in the United States raises a number of important constitutional questions—some of which have been answered by the U.S. Supreme Court, but many of which remain unresolved. This report will review the federal government's constitutional authority to enact the federal criminal prohibition on marijuana; highlight certain principles of federalism that prevent the federal government from mandating that states participate in enforcing the federal prohibition; consider unresolved questions relating to the extent to which state authorization and regulation of medical marijuana are preempted by federal law; and assess what obligations, if any, the U.S. Department of Justice (DOJ) has to investigate and prosecute violations of the federal prohibition on marijuana. Prior to considering the significant constitutional questions associated with the interplay between state and federal laws in the context of medical marijuana, the following section provides a description of the CSA and a brief discussion of common characteristics found within the wide variety of state medical marijuana laws that have been enacted across the country. Enacted in 1970, the CSA establishes a statutory framework through which the federal government regulates the lawful production, possession, and distribution of controlled substances. The CSA places various plants, drugs, and chemicals (such as narcotics, stimulants, depressants, hallucinogens, and anabolic steroids) into one of five schedules based on the substance's medical use, potential for abuse, and safety or dependence liability. Further, the act requires persons who handle controlled substances or listed chemicals (such as drug manufacturers, wholesale distributors, doctors, hospitals, pharmacies, and scientific researchers) to register with the Drug Enforcement Administration (DEA) in DOJ, which administers and enforces the CSA. Registrants must maintain detailed records of their respective controlled substance inventories, as well as establish adequate security controls to minimize theft and diversion. Marijuana is currently categorized as a Schedule I controlled substance, and is therefore subject to the most severe restrictions contained within the CSA. Schedule I drugs have "a high potential for abuse" and "no currently accepted medical use in treatment in the United States," and lack "accepted safety for use of the drug [] under medical supervisions." Pursuant to the CSA, the cultivation, distribution, or possession of marijuana is a federal crime. Although various factors contribute to the ultimate sentence received, the mere possession of marijuana generally constitutes a misdemeanor subject to up to one year imprisonment and a minimum fine of $1,000. The cultivation or distribution of marijuana, or the possession of marijuana with the intent to distribute, on the other hand, is subject to more severe penalties. Such conduct generally constitutes a felony subject to as much as five years imprisonment and a fine of up to $250,000. Although individuals who use medical marijuana in compliance with state law are still in violation of federal law and subject to prosecution by federal authorities at any time, the Obama Administration has announced an informal policy that suggests a federal prosecution in that situation would be unlikely. In an October 19, 2009, memorandum, Deputy Attorney General David W. Ogden provided guidance to federal prosecutors in states that have authorized the use of medical marijuana. Citing a desire to make "efficient and rational use of its limited investigative and prosecutorial resources," the memorandum stated that while the "prosecution of significant traffickers of illegal drugs, including marijuana … continues to be a core priority," federal prosecutors "should not focus federal resources [] on individuals whose actions are in clear and unambiguous compliance with existing state laws providing for the medical use of marijuana." The memorandum made clear, however, that "this guidance [does not] preclude investigation or prosecution, even where there is clear and unambiguous compliance with existing state law, in particular circumstances where investigation or prosecution otherwise serves important federal interests." Responding to an increase in the "commercial cultivation, sale, distribution, and use of marijuana for purported medical purposes," DOJ released a subsequent memorandum in 2011 drawing a clear distinction between the potential prosecutions of individual patients who require marijuana in the course of medical treatment and "commercial" dispensaries. After noting that several jurisdictions had recently "enacted legislation to authorize multiple large-scale, privately operated industrial marijuana cultivation centers," DOJ attempted to clarify the scope of the Ogden Memorandum: The Ogden memorandum was never intended to shield such activities from federal enforcement action and prosecution, even where those activities purport to comply with state law. Persons who are in the business of cultivating, selling or distributing marijuana, and those who knowingly facilitate such activities, are in violation of the [CSA] regardless of state law. Consistent with resource constraints and the discretion you may exercise in your district, such persons are subject to federal enforcement action, including potential prosecution. The memorandum clearly communicated that individuals operating or "facilitating" medical marijuana dispensaries, even if operated in compliance with state law, continue to be targets for federal prosecution. As a result, the last two years have seen a reported increase in the number of federal DEA raids on dispensaries and marijuana farms and the subsequent prosecutions of those who own and operate marijuana distribution facilities. Additionally, a number of states have abandoned legislative proposals to expand their medical marijuana programs, at least partly as a result of warnings from U.S. Attorneys that the DOJ will "vigorously" enforce the CSA against those who participate in the unlawful manufacturing or distribution of marijuana, regardless of whether such activity is licensed under state law. Forty-eight states and the District of Columbia have criminalized the recreational use of marijuana. However, beginning with California in 1996, a growing number of states have decriminalized the use of marijuana for medicinal purposes or exempted qualified users from sanctions imposed under state law. Today, 18 states and the District of Columbia have enacted provisions that, in various ways, exempt qualified individuals from state criminal prosecution and various state civil penalties for marijuana-related offenses. Although these laws vary widely in their approaches to medical marijuana, there are a number of common characteristics that appear to adhere to these laws. First, in order for an individual to legally use medical marijuana, the drug must have been recommended by a physician for use in treating a diagnosed medical condition. All states but California require that this recommendation be in writing. Most states also require potential users to register with the state. Upon registration, states will often provide the user with a registration card so that the individual can be identified as a qualified user of medical marijuana. Additionally, all states but California limit the quantity of marijuana that a patient may possess at any one time, and most states have laws limiting the manner and place in which a qualified individual can use the drug. Although these 19 jurisdictions have established a scheme by which qualified individuals may legally possess and use marijuana for medicinal purposes under state law, qualified users in some jurisdictions lack a legal avenue to obtain adequate quantities of the drug. Some states permit users to grow their own marijuana, while others license third-party private persons or entities to cultivate and distribute the drug to qualified individuals through state-licensed and -regulated dispensaries. California has also authorized patients and caregivers to collectively grow marijuana in "cannabis cooperatives." In those states where supply is limited, however, many medical marijuana users are forced to acquire the marijuana they are permitted to possess and use through the black market. The unique inconsistencies between federal and state approaches to medical marijuana give rise to a series of important constitutional questions. First, is it within Congress's power to prohibit the production, possession, and distribution of marijuana? Second, to what extent can the federal government direct states to adopt similar laws or enforce the federal prohibition? Third, to what extent are state attempts to authorize and regulate medical marijuana preempted by federal law? And finally, what obligation, if any, does DOJ have to enforce the federal prohibition? The U.S. Supreme Court considered the reach of Congress's Commerce Clause authority and the constitutionality of the CSA in Gonzales v. Raich . Raich involved a challenge to the federal marijuana prohibition brought by Angel Raich and Diane Monson after agents of the federal DEA seized and destroyed marijuana plants that Monson had been cultivating for medical purposes consistent with California law.