STUDY OBJECTIVE: The synthetic cathinone 3-methylmethcathinone (3-MMC, or metaphedrone) has recently gained popularity. We studied the numbers of 3-MMC poisonings over time and the clinical effects following poisonings with 3-MMC. METHODS: We performed a retrospective study on the numbers of self-reported 3-MMC poisonings to the Dutch Poisons Information Center (DPIC) from 2013 to June 2021. For poisonings reporting 3-MMC only, the symptoms were extracted and the Poisoning Severity Score (PSS) was determined. From 2016 to June 2019, a prospective cohort study on poisonings reporting only 3-MMC was performed, in which details on the clinical courses were collected through telephone interviews. RESULTS: From 2013 to June 2021, the DPIC was consulted on 184 3-MMC poisonings. The number of poisonings increased from 1 in 2013 to 70 in the first half of 2021. In 84 poisonings with only 3-MMC (46%), sympathomimetic symptoms were commonly reported, including tachycardia (n=29, 35%), hypertension (n=17, 20%), and agitation (n=16, 19%). The initial PSS was usually minor (n=37, 44%) to moderate (n=39, 46%). Five patients (6%) experienced severe effects, including 3 patients experienced severe hypertension (systolic blood pressure >180 mmHg; n=3) and nonfatal cardiac arrest (n=1). Sympathomimetic symptoms (n=8) were also reported in the prospective cohort study. The percentage of moderate poisonings increased (n=6, 75%), and 1 (13%) severe poisoning was observed. Analytical confirmation of 3-MMC exposure was performed in 2 cases. CONCLUSION: The number of 3-MMC poisonings reported to the DPIC has increased over time. Most poisonings with 3-MMC resulted in moderate toxicity and involved sympathomimetic effects, while severe effects were observed in 5 cases.