Background and Aims: Drug checking services provide people who use drugs with chemical analysis results of their drug samples while simultaneously monitoring the unregulated drug market. We sought to identify and synthesize literature on the following domains: (a) the influence of drug checking services on the behaviour of people who use drugs; (b) monitoring of drug markets by drug checking services; and (c) outcomes related to models of drug checking services. Methods: Systematic review. A systematic literature search was conducted in MEDLINE, Embase, Cochrane Central Register of Controlled Trials, Cochrane Database of Systematic Reviews, PsycINFO, Scopus, Web of Science and Dissertations and Theses Global. Eligible studies were peer‐reviewed articles and conference abstracts or grey literature, published in any language since 1990 and including original data on the domains. We assessed risk of bias for quantitative peer‐reviewed articles reporting on behaviour or models of drug checking services using National Institutes of Health tools. Results: We screened 2463 titles and abstracts and 156 full texts, with 90 studies meeting inclusion criteria. Most ( n = 65, 72.2%) were from Europe and used cross‐sectional designs ( n = 79, 87.7%). Monitoring of drug markets by drug checking services ( n = 63, 70%) was the most reported domain, followed by the influence of drug checking services on behaviour ( n = 31, 34.4%), including intent to use, actual use and disposal of the drug, and outcomes related to models of drug checking services ( n = 17, 18.9%). The most common outcome measures were detection of unexpected substances ( n = 50, 55.6%), expected substances ( n = 44, 48.9%), new psychoactive substances ( n = 40, 44.4%) and drugs of concern ( n = 32, 36.5%) by drug checking services. Conclusions: Drug checking services appear to influence behavioural intentions and the behaviour of people who use drugs, particularly when results from drug checking services are unexpected or drugs of concern. Monitoring of drug markets by drug checking services is well established in Europe, and increasingly in North America. Concerns about drug contents and negative health consequences facilitate the use of drug checking services; lack of concern; trust in drug sellers; lack of accessibility of drug checking services; and legal and privacy concerns are barriers to use.

Background: The North American opioid overdose crisis is driven in large part by the presence of unknown psychoactive adulterants in the dynamic, unregulated drug supply. We herein report the first detection of the psychoactive veterinary compound xylazine in Toronto, the largest urban center in Canada, by the city’s drug checking service. Methods: Toronto’s Drug Checking Service launched in October 2019. Between then and February 2021, 2263 samples were submitted for analysis. The service is offered voluntarily at harm reduction agencies that include supervised consumption services. Samples were analyzed using gas chromatography–mass spectrometry or liquid chromatography-high resolution mass spectrometry. Targeted and/or untargeted screens for psychoactive substances were undertaken. Results: In September 2020, xylazine was first detected by Toronto’s Drug Checking Service. Among samples analyzed from September 2020 to February 2021 expected to contain fentanyl in isolation (610) or in combination with methamphetamine (16), xylazine was detected in 46 samples (7.2% and 12.5% of samples, respectively). Samples were predominantly drawn from used drug equipment. Three of the samples containing xylazine (6.5%) were associated with an overdose. Conclusion: We present the first detection of xylazine in Toronto, North America’s fourth-largest metropolitan area. The increased risk of overdose associated with use of xylazine and its detection within our setting highlights the importance of drug checking services in supporting rapid responses to the emergence of potentially harmful adulterants. These data also highlight the clinical challenges presented by the dynamic nature of unregulated drug markets and the concomitant need to establish regulatory structures to reduce their contribution to overdose morbidity and mortality.