© 2018 Koninklijke Brill NV, Leiden, The Netherlands. Legal tensions are growing as more jurisdictions move towards legal regulation of the cannabis market in contravention of the obligation under UN drug control treaties to limit cannabis exclusively to medical and scientific purposes. Reaching a global consensus to amend the conventions does not appear to be a viable political option in the foreseeable future. Amongst the limited options not requiring consensus, inter se modification - based on article 41 of the Vienna Convention on the Law of Treaties - provides a useful safety valve for collective action to adjust a treaty regime arguably frozen in time. Restrictions imposed on inter se modification require a clear commitment to the original treaty aim to promote the health and welfare of humankind and to the original treaty obligations vis-à-vis states not party to the agreement. A coordinated collective response has benefits compared to a chaotic scenario of multiple unilateral reservations and questionable re-interpretations.