["Addiction, EarlyView. ", "\nAbstract\n\nBackground and aims\nThe market for new psychoactive substances (NPS) is highly dynamic, with hundreds of substances emerging on the European drug market in the last decade. 3‐methylmethcathinone (3‐MMC), a synthetic cathinone and isomer of mephedrone (4‐MMC), gained popularity in the Netherlands due to its psychostimulant effects and initial legal status. The present study aimed to systematically examine developments in the Dutch 3‐MMC market between 2012 and 2024, based on the Drugs Information and Monitoring System (DIMS) data, to describe trends in pricing, sources of purchase, chemical composition, user‐reported effects and demographic and contextual use patterns.\n\n\nDesign and setting\nObservational study using drug checking data collected from 2012 to 2024 through DIMS, a national network of 33 drug checking services operating throughout the Netherlands.\n\n\nCases\nIn total, 3437 samples sold as 3‐MMC were submitted to DIMS for analysis and were included in the study. Of these, 2627 samples were chemically analysed.\n\n\nMeasurements\nNumber of samples sold as 3‐MMC submitted for drug checking, price, source of purchase, drug composition, self‐reported effects in relation to drug composition, and setting of use.\n\n\nFindings\nThe number of samples sold as 3‐MMC submitted to DIMS each year increased from n = 1 in 2012 to n = 1853 in 2024. Following the scheduling of 3‐MMC in 2021, median prices doubled (from €10 during the period 2019–2021 to €20 in 2022) and the reported source of purchase shifted towards dealer (from approximately 50% of samples before scheduling to more than 90% after scheduling). Chemical composition of the analysed samples changed significantly (Cramér's V = 0.49, P < 0.01). Since 2022, samples sold as 3‐MMC increasingly contained a variety of substances. By 2024, only 10% of samples still contained 3‐MMC, compared with 85% in 2021, whereas substitutes such as 2‐methylmethcathinone (2‐MMC) and 3‐chloromethcathinone (3‐CMC), as well as more potent compounds including di(methyl)pentylone (DMP) and N‐ethylnorpentedrone (NEP) became more prevalent. Self‐reported effects (noted for 376 samples) were qualitatively similar for samples containing 2‐MMC and 3‐CMC compared with 3‐MMC, whereas DMP and NEP were associated with more severe adverse reactions (e.g. anxiety, paranoia, cardiovascular disturbances, insomnia). Patterns of use shifted towards festivals and parties (around 50% and 25%, respectively), while home use (15%) remained notable in 2024.\n\n\nConclusions\nThe Dutch 3‐methylmethcathinone (3‐MMC) market changed substantially following scheduling in 2021, with more samples sold as 3‐MMC submitted for drug checking. By 2024, only 10% of submitted samples contained 3‐MMC and structurally related substitutes with greater health risks were increasingly detected.\n\n"]