Cannabis is subject to international control by the 1961 Single Convention on Narcotic Drugs, as amended in 1972 (http://www.incb.org/incb/convention_1961.html), and it is also affected by the 1988 Convention against Illicit Traffic in Narcotic Drugs and Psychotropic Substances (http://www.incb.org/incb/convention_1988.html). Because a basic principle of these conventions is that legitimate use of substances covered by them should be limited to medical and scientific purposes, they have been an effective block to efforts at a national or sub-national level to move in any way to a regulatory system of control that aims to regulate use so as to minimize social and health harm.
While in principle these Conventions can be amended, this is not a practical possibility at the present time. An alternative path is for like-minded states to adopt a new Convention specifically devoted to cannabis. On the legal principle of “last in time” taking precedence, this can be argued to take precedence in and between those states adopting such a new convention (Room et al., 2008: 159-162)
A precedent for a new convention covering a single psychoactive substance is the Framework Convention on Tobacco Control (FCTC; http://www.who.int/fctc/text_download/en/index.html). This convention was negotiated under WHO auspices, was adopted in 2003, and came into force in 2005. It has been proposed that cannabis might be added to this Convention (Spivack, 2004), but this would require an amendment process which is also not presently a practical possibility. The alternative, which is explored here, is to adopt a new convention, which might well be modelled on the tobacco convention.