Estimates of people who inject drugs (PWID) are crucial to ensure that dedicated services are adequately maintained and to monitor the harm reduction targets set by international organisations.

We obtained pseudo-anonymized identifier information about PWID from two treatment and harm reduction services and a fieldwork study between February and April 2019. We estimated the number of PWID, defined as people who injected drugs in Brussels within the last 12 months, using capture-recapture methodology. We used log-linear modelling to analyse the overlap in the number of PWID in these three data sources. For every fitted model, we estimated the number of PWID in Brussels. The results were further used to estimate the number of PWID in Belgium through a multiplier method using two existing treatment registers.

After matching the three data sources, we identified 269 unique adult PWID in Brussels. Based on the selected model, we reached an estimated total population of adult PWID in Brussels of 703 (95 %CI 538–935), which means an estimated population prevalence of 0.8 PWID per 1000 adult inhabitants (95 %CI 0.6–1.0) in Brussels and between 0.7 PWID (95 %CI 0.5 – 0.9) and 0.8 PWID (95 %CI 0.5–1.0) per 1000 adult inhabitants in Belgium.

The results are crucial for the further development of treatment and harm reduction services in order to achieve the targets set by WHO on the provision of sterile needles and syringes, the reduction of new HCV infections and related mortality.