Understanding how many people inject drugs is critically important for the effective provision of public health and harm reduction services. However, the severe stigma that is often attached to injecting drug use, and illicit nature of this behaviour, means that it is often difficult to have a clear picture of how many people inject drugs in a given nation or region.1 Without this basic population number, it is also difficult to know how many people who inject drugs are living with HIV or hepatitis C virus (HCV), or to know how well HIV and HCV prevention interventions such as needle and syringe programs and opioid substitution therapy are reaching people in need.

We sought to review evidence globally to estimate:
1. The number of people who inject drugs
2. Women and young people as a proportion of people who inject drugs
3. HIV and HCV antibody (anti-HCV) prevalence among people who inject drugs
4. Coverage of needle and syringe programs (NSP) and opioid substitution therapy (OST)

For each of these, we calculated national, regional and global estimates. Estimates were calculated from data presented in peer-reviewed literature and reports from government departments, non-government agencies, and international organisations such as UNAIDS and the Global Fund.