Objectives Illegal drug use continues to be a major threat to community health and safety. We used international drug surveillance databases to assess the relationship between multiple long-term estimates of illegal drug price and purity.

Design We systematically searched for longitudinal measures of illegal drug supply indicators to assess the long-term impact of enforcement-based supply reduction interventions.

Setting Data from identified illegal drug surveillance systems were analysed using an a priori defined protocol in which we sought to present annual estimates beginning in 1990. Data were then subjected to trend analyses.

Main outcome measures Data were obtained from government surveillance systems assessing price, purity and/or seizure quantities of illegal drugs; systems with at least 10 years of longitudinal data assessing price, purity/potency or seizures were included.

Results We identified seven regional/international metasurveillance systems with longitudinal measures of price or purity/potency that met eligibility criteria. In the USA, the average inflation-adjusted and purity-adjusted prices of heroin, cocaine and cannabis decreased by 81%, 80% and 86%, respectively, between 1990 and 2007, whereas average purity increased by 60%, 11% and 161%, respectively. Similar trends were observed in Europe, where during the same period the average inflation-adjusted price of opiates and cocaine decreased by 74% and 51%, respectively. In Australia, the average inflation-adjusted price of cocaine decreased 14%, while the inflation-adjusted price of heroin and cannabis both decreased 49% between 2000 and 2010. During this time, seizures of these drugs in major production regions and major domestic markets generally increased.

Conclusions With few exceptions and despite increasing investments in enforcement-based supply reduction efforts aimed at disrupting global drug supply, illegal drug prices have generally decreased while drug purity has generally increased since 1990. These findings suggest that expanding efforts at controlling the global illegal drug market through law enforcement are failing.