By Ayden Scheim, Nazlee Maghsoudi, Zack Marshall, Siobhan Churchill, Carolyn Ziegler, Dan Werb - BMJ Journals

An estimated 271million people used an internationally scheduled (‘illicit’) drug in 2017, corresponding to 5.5% of the global population aged 15 to 64. Despite decades of investment, policies aimed at reducing supply and demand have demonstrated limited effectiveness. Moreover, prohibitive and punitive drug policies have had counterproductive effects by contributing to HIV and hepatitis C transmission, fatal overdose, mass incarceration and other human rights violations and drug market violence. As a result, there have been growing calls for drug law reform and in 2019, the United Nations Chief Executives Board endorsed decriminalisation of drug use and possession. Against this backdrop, as of 2017 approximately 23 countries had implemented de jure decriminalisation or legal regulation of one or more previously illegal drugs.

A wide range of health and social outcomes are affected by psychoactive drug production, sales and use, and thus are potentially impacted by drug law reform. Nutt and colleagues have categorised these as physical harms (e.g., drug-related morbidity and mortality to users, injury to non-users), psychological harms (e.g., dependence) and social harms (e.g., loss of tangibles, environmental damage). Concomitantly, a diverse and sometimes competing set of goals motivate drug policy development, including ameliorating the poor health and social marginalisation experienced by people who use drugs problematically, shifting patterns of use to less harmful products or modes of administration, curtailing illegal markets and drug-related crime and reducing the economic burden of drug-related harms.

Given ongoing interest by states in drug law reform, as well as the recent position statement by the United Nations Chief Executives Board endorsing drug decriminalisation, a comprehensive understanding of their impacts to date is required. However, the scientific literature has not been well-characterised, and thus the state of the evidence related to these heterogeneous policy targets remains largely unclear. Systematic reviews, including two meta-analyses, are narrowly focussed on adolescent cannabis use. Dirisu et al. found no conclusive evidence that cannabis legalisation for medical or recreational purposes increases cannabis use by young people. In the two meta-analyses, Sarvet et al. found that the implementation of medical cannabis policies in the USA did not lead to increases in the prevalence of past-month cannabis use among adolescents and Melchior et al found a small increase in use following recreational legalisation that was reported only among lower-quality studies

Given increasing interest in quantifying the impact of drug law reform, as well as a lack of systematic assessment of outcomes beyond adolescent cannabis use to date, we conducted a systematic review of original peer-reviewed research evaluating the impacts of (a) legal regulation and (b) drug decriminalisation on drug availability, use or related health and social harms. Our primary aim is to characterise studies with respect to metrics and indicators used. The secondary aim is to summarise the findings and methodological quality of studies to date.