Cannabis policies are changing in some countries. This may have consequences that extend beyond cannabis-specific outcomes, such as an impact on the consumption patterns of other substances. Changes in cannabis policies may also influence policy responses to other drugs, as countries re-assess the balance between law enforcement and public health objectives. If this happens, it could have important health and social consequences, especially in those countries where a 'war on drugs' policy perspective has inhibited investment in evidence based responses in areas such as treatment and harm reduction. The burden of disease associated with opioid use for example is large and this is an area in which treatment and harm reduction have been shown to deliver benefits. Thus if the changes in cannabis policies result in a greater willingness to invest in effective interventions for other drugs, the potential net health gains could be considerable. On the other hand, if cannabis policy changes are associated with an increase in health risk behaviours, such as driving under the influence or increased use of harmful substances such as tobacco, then significant increased health costs could result. To date most attention has been focused on recent cannabis sales liberalisation in the Americas, but experiences from elsewhere are also informative. In Europe, for example, moves towards decriminalisation of drug possession are resulting in lower rates of incarceration and arguably have reduced barriers to treatment uptake. Robust monitoring and assessment of the impact of these different policy changes is crucial to evaluating and understanding their results. It is important that such monitoring is international in scope, is not limited to issues around the use of cannabis only, and considers the interactions that may exist between cannabis policies and the approaches taken to other substances.