Marijuana remains a Schedule I drug under the federal Controlled Substances Act, making it illegal anywhere in the United States.
Article VI, Clause 2 of the United States Constitution, known as Supremacy of National Law , establishes the U.S. Constitution, U.S. Treaties, and Federal Statutes as "the supreme law of the land." The text decrees these to be the highest form of law in the U.S. legal system, and mandates that all state judges must follow federal law when a conflict arises between federal law and either the state constitution or state law of any state.
In Gonzales v. Raich, the Supreme Court found that a Congressional act to prohibit marijuana outright is valid even when it conflicts with state laws that allow exceptions for medical necessity because banning marijuana falls under Congress's authority to regulate interstate commerce under the Commerce Clause.
Furthermore, in California v. ARC America Corp, the court found (according to Wikipedia) that "if Congress expressly intended to act in an area, this would trigger the Supremacy Clause and nullify State action."