Treaty interpretation has long been a subject of interest for international legal scholars. However, it is only recently that advocates for drug policy reform have taken up these questions. This article examines the proposition put forward by several authors that a legally regulated market in cannabis may be permissible under the international drug control treaties if considered as a policy ‘experiment’. These authors contend that such measures conform to the general obligation of the Single Convention on Narcotic Drugs to limit uses of cannabis ‘strictly to medical and scientific purposes’. Reviewing this position using the formal methods set out in Articles 31 and 32 of the Vienna Convention on the Law of Treaties, we conclude the interpretation proposed is untenable. While we share with these authors the objective of wider drug policy reform, we find the arguments supporting this position weak, and based on absent, flawed or incomplete interpretive methodology.