["Addiction, EarlyView. ", "\nAbstract\n\nAims\nThis study examined the use of a drug checking service by drug sellers in British Columbia, Canada, to assess motivations for service use and substances intended for distribution, focusing on risks associated with an unstable illicit drug market.\n\n\nDesign\nA retrospective analysis was conducted using data from an anonymous intake survey and drug checking results. The survey captured frequency of service use, expected substances and reasons for service access.\n\n\nSetting\nThe study was set in Victoria, British Columbia, where participants accessed a free confidential drug checking service between January 2021 and December 2024.\n\n\nParticipants\nA total of 2332 drug sellers were included in the analysis, accounting for 16.2% of all service users. They submitted 5528 samples (21.6% of total samples), averaging 2.4 samples per visit, compared to 1.7 among non‐sellers.\n\n\nMeasurements\nThe primary outcomes measured included the frequency of drug check use, the expected and actual composition of the samples, pre‐ or post‐consumption analysis and the reasons for engaging with the service. Secondary outcomes focused on the reported potency and the presence of unexpected substances.\n\n\nFindings\nA statistically significant portion of sellers continuously used the service, particularly for opioid samples, which comprised 69.2% of seller submissions compared with 35.1% among non‐sellers [X2 (1, n = 20 864) = 2055.57, P < 0.001]. Sellers reported using the service to mitigate risks associated with the illicit drug market, including overdose, showing a proactive interest in ensuring the safety of their customers. Despite greater confidence in the composition of their products, sellers were less likely than non‐sellers to have their samples confirmed as “expected” [38.4% vs. 51.9%; X2 (1, n = 12 520) = 313.76, P < 0.001]; unexpected substances were detected in 44.8% of their samples. Notably, protective strategies employed by sellers were identified, demonstrating the role of drug checking as a supply‐focused harm reduction intervention.\n\n\nConclusions\nIn Victoria, British Columbia, drug sellers appear to be increasingly using drug checking services as a preventive measure, positioning them as potential partners in harm reduction efforts.\n\n"]