Population aging and apparent decline in initiation of injecting drug use in Australia

Background: The age of participants in cross-sectional surveys of people in Australia who inject drugs is increasing. We aimed to characterize this change in terms of age at first injection, time since first injection, and potential cohort effects.

Methods: We used 20 years (2000-2019) of annual repeat cross-sectional survey data from the Australian Illicit Drug Reporting System, in which people who regularly inject drugs participated. We estimated quantile regression models to investigate trends in median age, time since injecting initiation, and age at injecting initiation. 

Results: In total, 18,163 people participated (66% man/male). The median age of participants increased across the survey period from 28 years (interquartile range [IQR]: 22-35) in 2000 to 44 years (IQR: 38-50) in 2019. Initiation of injecting drug use increased steadily from the 1970s, peaking in the 1990s before sharply declining after 2000. Consequently, median time since injecting initiation increased across the survey period from 8 years (IQR: 4-15) to 24 years (IQR: 18-31). By calendar year of injection initiation, the median age at initiation has increased from 18 years (IQR: 16-22) in 1995 to 28 years (IQR: 21-40) in 2015. Compared to men, women were younger across the survey period and typically commenced injecting at a younger age.

Conclusions: It appears that the increasing age of people in Australia who inject drugs is driven by a large group of people who initiated injecting in the 1990s and continue to inject. Age at injecting initiation also increased, albeit at a smaller magnitude. Our findings have implications for health service delivery to people who inject drugs, with an increase in age likely to be accompanied by a rise in chronic health conditions and an increase in injecting duration potentially resulting in a higher incidence of injecting-related injuries and diseases.