Substances from unregulated drug markets - A retrospective data analysis of customer-provided samples from a decade of drug checking service in Zurich (Switzerland).

Background: Drug checking services (DCS) are harm reduction interventions for people who consume illicit substances. Unregulated drug markets lead to samples with unexpected and variable contents. A retrospective data analysis of Zurich's DCS was performed to determine the nature of these samples.

Methods: This study aims to investigate the qualitative and quantitative properties of 16,815 customer-provided psychoactive drug samples analyzed chemically through the DCS in Zurich from 1 January 2011 to 31 December 2021. The main analytical method utilized for characterizing these substances was high-performance liquid chromatography and gas chromatography-mass spectrometry. Data sets are summarized using descriptive statistics.

Results: There was a 2.5-fold increase in the number of tested samples over the past decade. An overall proportion of 57.9% (weighted mean) of samples within our database demonstrates unexpected analytical findings and additional low sample contents during the observation period. Substantial differences in quality and quantity between substance groups were detected and an increase of sample quality and content over time was demonstrated.

Conclusions: Chemical analysis reveals that over half of substances acquired from unregulated drug markets analyzed through DCS in Zurich are with low qualitative and quantitative properties, which may expose users to risks. Based on longitudinal analyses over a decade, this study contributes to the body of evidence that DCS may potentially manipulate unregulated drug markets towards providing better quality substances, as well as may stabilize these markets over time. The necessity for drug policy changes to make this service accessible in further settings was highlighted, as DCS still often take place in legal grey zones.