Last week, as Mike Riggs noted a few days ago, the president of the International Narcotics Control Board (INCB), Raymond Yans, argued that the U.S. government's treaty obligations preclude Colorado and Washington from legalizing marijuana. Responding to passage of Colorado's Amendment 64 and Washington's Initiative 502, Yans said "these developments are in violation of the international drug control treaties." That does not appear to be true, and even if it were the U.S. Constitution would bar the federal government from forcing states to ban marijuana.

In a November 15 press release from the INCB, Yans claims the Single Convention on Narcotic Drugs, adopted in 1961 and amended in 1972, requires the U.S. government to override the drug policy choices made by voters in Colorado and Washington:

The limiting of the use of cannabis to medical and scientific purposes is laid out in the 1961 Single Convention on Narcotic Drugs, which was agreed to by 185 States, who by  consensus decided to place cannabis under control and limit its use to  medical purposes....

For the international drug control system to function effectively, to achieve its aim of ensuring availability of drugs for medical purposes while preventing their abuse, the conventions must be universally adhered to and implemented by all States. In this regard, Mr. Yans stressed that national laws, policies and practices in drug abuse prevention and control should be fully aligned with the conventions. He further emphasized that States Parties have an obligation under the Conventions to ensure their full compliance with the conventions within their entire territory, including federated states and/or provinces.

Mr. Yans recognized the commitment of the Government of the United States to resolve the contradiction between the federal and state levels in the implementation of that country's obligations under the drug control conventions. The INCB President requested the Government of the United States to take the necessary measures to ensure full compliance with the international drug control treaties within the entire territory of the United States, in order to protect the health and well-being of its citizens. 

The "necessary measures" Yans is demanding are clearly unconstitutional. The Constitution's Supremacy Clause says "this Constitution, and the Laws of the United States which shall be made in pursuance thereof; and all treaties made, or which shall be made, under the authority of the United States, shall be the supreme law of the land." But it is also clear, as Jonathan Caulkins and three other drug policy scholars note in Marijuana Legalization: What Everyone Needs to Know, that "the Constitution does not allow the federal government either to order state governments to create any particular criminal law or to require state and local police to enforce federal criminal laws." Hence a treaty that purported to require such legal subjugation would not be "under the authority of the United States," and any act of Congress aimed at dictating state drug laws would not be "made in pursuance" of the Constitution.

Furthermore, the Single Convention on Narcotic Drugs says a signatory's obligation to enact criminal penalties for the nonmedical production, possession, and distribution of marijuana is "subject to its constitutional limitations." Patrick Gallahue of the Open Society Global Drug Policy Program says:

The penal provisions of the '61 convention includes the caveat: "subject to the constitutional limitations of a Party, its legal system and domestic law." How does that interact with the 10th Amendment to the U.S. Constitution, which states, "The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people"?...

There is an argument that [America's] unique situation regarding states' rights allows one of these states to do this [i.e., legalize marijuana without violating the treaty]. 

Gallahue adds that "there is always the option for the United States to withdraw from the '61 convention and re-enter with reservations," as Bolivia recently did so it could legalize the traditional use of coca. But even if that never happens, and even if Yans' reading of the treaty is correct, the federal government simply does not have the authority to do what he wants. 

Addendum: For a thorough and incisive critique of the view that international treaties can expand the legislative powers of Congress beyond those enumerated by the Constitution (an idea that the Supreme Court endorsed in the 1920 case Missouri v. Holland), see this 2005 Harvard Law Review article by Georgetown law professor Nicholas Quinn Rosenkranz.

[Thanks to Bruce Majors for the link to the INCB press release.]