Background: Illicit drug quality is notoriously unpredictable, with street samples showing variability in potency and concealed adulterants. Amid the ongoing opioid crisis in the United States, recent international detections of synthetic opioids, sometimes marked as heroin, underscore the associated health risks and the need for reliable drug composition monitoring. Shifts in drug availability, for example, following the Taliban’s 2022 opium cultivation ban, further amplify uncertainties in heroin quality. Despite the importance of timely, representative data, current monitoring does not fully capture the retail market. This study characterizes the chemical profile of street heroin in Belgium, analysing its composition while integrating the perspectives of people who use heroin.

Methods: This 2023 multicentre mixed-method study involves a paper-based questionnaire by people who use heroin, covering topics like current heroin use, poly-drug use, purchasing details, and experiences of adverse effects after consumption. Participants also donated a 100 mg heroin sample, which was subsequently analysed using qualitative and quantitative analytical methods to determine its composition. Secondary data on heroin purity from 2020 to 2023, obtained from police, customs, and toxicological laboratories, supplemented the study.

Results: Analysis of 128 samples, originating from three different cities, revealed a mean heroin purity of 13.0%, a range: 0.2–56.5%, and minimal inter-city differences. A limited number of adulterants were detected, including acetaminophen, caffeine, cocaine, bromazepam, pseudococaine, and ketamine. None of the samples contained synthetic opioids or “nitazene” 2-benzylbenzimidazole analogues. Further, 8% of the respondents (n = 9) reported experiencing unattended effects—such as nausea, itchiness, and local irritation—and the mean perceived quality of the heroin was 5.5 out of 10.

Conclusion: This study provides a timely characterization of street heroin in Belgium, documenting variations in purity, pricing, and adulterants, while strongly indicating the absence of fentanyl and other synthetic opioids in 2023. The low perceived quality of heroin, self-reported unexpected effects, and the dynamic nature of the unregulated supply nevertheless highlight the need for sustained monitoring and continued harm-reduction efforts in collaboration with grassroots organizations, particularly in the context of an evolving European heroin market.