Wastewater-based epidemiology (WBE) is proving to be a promising tool to back-estimate illicit drug market sizes. Within the framework of the Sewage analysis CORe group Europe (SCORE) network, influent wastewater (IWW) samples were analysed for metabolites and parent compounds of illicit drugs to determine consumption estimates for amphetamine, cocaine, 3,4-methylenedioxymethamphetamine (MDMA) and methamphetamine. Between 2015 and 2021 measured concentrations of illicit drugs were translated into amounts of pure compound by considering flow rates, population numbers and most recent excretion factors. WBE-derived concentrations of these stimulants together with other data sources available to the EMCDDA, notably annual average retail purity and price data, or other information such as city-level data from drug checking services, were used to back-estimate illicit drug market sizes. From 2015 to 2021, annual drug estimates of consumed pure and consumed street-level drugs were available for 1 846 and 891 European cities, respectively. However, due to the scarcity of comprehensive information on drug purity and pricing, market size values could be calculated for only 385 locations in this period. This highlights the need for up-to-date and representative information on drug price and purity, and increased standardisation of sampling strategies to obtain more data.
Additionally, this study provides a comprehensive overview of some intrinsic limitations associated with the back-estimation of drug market shares based on WBE data. Since its inception, there has been a large amount of fundamental research conducted within the field of WBE, as well as studies focused on harmonising analytical methodologies. However, there are still some knowledge gaps that require further investigation. Nevertheless, WBE could complement other epidemiological data sources that have been used to calculate drug market sizes (e.g., health interview surveys, seizures, the European Web Survey on Drugs). Finally, this investigation proposes recommendations for future WBE studies aiming to estimate drug market sizes.
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