While marijuana possession and distribution is a federal crime under the Controlled Substances Act (CSA) and the Supremacy Clause of the United States Constitution provides that federal law is the “supreme Law of the Land,”  that is not the entire story: The U.S. system of government is one of dual sovereignty where the states can and do serve as “laboratories of democracy.”
The question of federal preemption is a question of Congressional intent. The CSA makes it clear it only preempts state laws under very limited circumstances. 21 U.S.C. 903 says it is not intended to preempt the field of drug laws if “there is a positive conflict” between state and federal law “so that the two cannot consistently stand together.” Courts have generally held that a state law is only preempted by the CSA if it is “physically impossible” to comply with both state and federal law or if the state law stands as an obstacle to the CSA. Neither is the case with carefully crafted state medical marijuana programs.
A state law — or a portion of it — would be preempted under impossibility preemption if it required someone to violate federal law. For this reason, effective medical marijuana laws do not require state workers to grow or dispense marijuana in violation of federal law; they just regulate private individuals who choose to do so. Requiring someone to break federal law is quite different from allowing and regulating conduct under state law.
When discussing preemption, it’s important not to forget about the Tenth Amendment. The federal government is free to enforce its own marijuana laws, but requiring state agents to enforce federal laws would be unconstitutional commandeering of a state’s resources. As one court noted:
It is of considerable consequence that it is Arizona's attempt at partial decriminalization with strict regulation that makes the AMMA vulnerable ... This view, if successful, highjacks Arizona drug laws and obligates Arizonans to enforce federal prescriptions that categorically prohibit the use of all marijuana. The Tenth Amendment's “anti-commandeering rule” prohibits Congress from charting that course.
The federal government has never alleged in court that federal laws preempt state medical marijuana or legalization and regulation laws. In fact, the Department of Justice (DOJ) argued in favor of dismissing a lawsuit claiming Arizona’s medical marijuana law was preempted. That suit was dismissed.
Federal Policy Should Not Deter States From Reforming Their Own Laws
Since late 2014, Congress has approved a rider to the annual Justice Department appropriations bill that provides that the funds may not be used to interfere with the implementation of state medical marijuana laws. Members of Congress have proposed a similar amendment to prevent interference with state-legal adult-use marijuana businesses, but it has not gotten a vote in recent years. Legislation was also put forth to make state-legal marijuana activities legal under federal law.
On January 4, 2018, U.S. Attorney General Jeff Sessions rescinded the 2013 Cole Memo that advised against targeting state-legal marijuana providers. Sessions’ reversal of the Cole Memo was deeply unpopular. It spurred significant bipartisan criticism, and a number of major editorials in opposition. Polls show more than 70 percent of Americans believe the federal government should not use its limited resources to interference with state-legal marijuana businesses. In April 2018, Pres. Donald Trump told U.S. Senator Cory Gardner (R-CO) that he would support a federalism-based legislative solution, and that the marijuana industry in Gardner’s home state would not be targeted.
It is up to individuals to decide whether they want to take the risk of breaking federal law, and many individuals and businesses are already doing so. What state lawmakers can and should do is remove the barriers to relief that their state law poses to humane, sensible marijuana policies. The vast majority of states have opted to chart a different course than the federal government and they will continue to do so. With growing state pressure, federal policy will likely change soon.
 U.S. Constitution Article VI, cl. 2.
 See: Murphy v. NCAA, No. 16–476, U.S. (2018) (“The PASPA provision at issue here—prohibiting state authorization of sports gambling—violates the anticommandeering rule. … [S]tate legislatures are put under the direct control of Congress. It is as if federal officers were installed in state legislative chambers and were armed with the authority to stop legislators from voting on any offending proposals. A more direct affront to state sovereignty is not easy to imagine.”); Printz v. United States, 521 U.S. 898 (1997); New York v. United States, 505 U.S. 144 (1992). See also: Gonzales v. Oregon, 546 U.S. 243, 270 (2006) (“the structure and limitations of federalism … allow the States great latitude under their police powers to legislate as to the protection of the lives, limbs, health, comfort, and quiet of all persons.”).
 White Mountain Health Center, Inc. v. Maricopa County, CV 2012-053585 (Arizona Superior Court, Maricopa County, 2012).
 Arizona v. United States, No. CV 11-1072-PHX-SRB, slip op. at 2 (D. Ariz. Jan. 1, 2012).
 See: Consolidated and Further Continuing Appropriations Act of 2015, Section 538, Pub. L. 113-235, 128 Stat. 2130 (2014) and U.S. v. Marin Alliance for Medical Marijuana, No. C 98-00086 CRB, decided October 19, 2015. (The rider also prevents the federal prosecution of individuals complying with state medical marijuana laws.)
 See: Carl Husle, “Trump’s New Policy on Marijuana Draws Bipartisan Fire,” New York Times, January 5, 2018.
 “Marijuana legalization support at all-time high,” CBS News, April 20, 2017.