Background Research on legal minors who inject drugs is rare and beset with methodological problems. We explore the potential of register data for researching this population and present illustrative analyses contributing to the knowledge base.

Method We obtained register data with national coverage from three high-income countries and assessed its analytic potential across two research themes. Descriptive analyses of the obtained data explored prevalence, mean age of debut, delay to accessing services, and treatment needs.

Results There were differences in register types and variables collected within and between each country. The analytical potential of register data was assessed as good. Prevalence of injecting debut before the age of 18 (IDU < 18) was 29% (Sweden), 23% (Switzerland), and 5%–9% (Wales). IDU < 18 took longer to access services in each country than those with debut as adults. Complex treatment needs were identified, including mental health, education, and housing.

Conclusion Register data is a viable method for better understanding this small, ‘hidden’ population, albeit with important caveats. Adolescent drug services may need to deliver or link to a range of treatment responses beyond drug use. Innovative outreach, and legal and policy change may be needed to ensure harm reduction access while injecting continues.