Estimating the size of crack cocaine users in France: Methods for an elusive population with high heterogeneity

Abstract Background Despite several sources corroborating an expanding market and increased visibility and greater diversity in users’ profiles, very little is known about the number of crack cocaine users in France. Method The estimates rely on a single data source capture–recapture method. Annual data are extracted from treatment centres nationwide. To account for heterogeneity, we use an innovative zero-truncated geometric, regression-based estimator controlling for individual and centre characteristics. We use the well-known Zelterman estimator as a benchmark. Results The number of crack cocaine users received in treatment centres increased dramatically, from 3388 in 2010 to 5143 in 2017 (+52%). The estimated number of crack cocaine users is believed to have tripled over the course of the same period (from 9775, 95% CI [8288–11530] to 28983 [24876–33766], respectively), with prevalences below 1‰, similar to other European countries. The coverage rate (observed number/estimated number of users) decreased in a similar fashion, indicating lower utilization. In particular, females and younger users are underestimated by data from treatment centres. Conclusion The prevalence of crack cocaine use is fairly low but steadily increasing. The diversity in users’ profiles is a challenge to prevention and public health policies that should expand their scope to a more inclusive perspective of what defines crack cocaine users. Our method overcomes several methodological issues (data sources, data linkage, heterogeneity) and can be easily applied to a wide range of settings.