From initiating injecting drug use to regular injecting: Retrospective survival analysis of injecting progression within a sample of people who inject drugs regularly

Background: The initiation of injecting drug use and the commencement of a pattern of regular injecting are key milestones in injecting careers. The progression from initiation to regular injecting is a poorly understood period in these careers. Methods: Cross-sectional baseline data from a sample of people who inject drugs regularly (N = 691), recorded the age at which participants initiated injecting drug use and the age they became regular (at least once per month) injectors. Survival analysis compared the rapidity of progression to regular injecting across sub-groups within the sample using bivariate log-rank testing and multivariable Cox regression. Results: Half of all participants progressed to regular injecting within 1 year of initiation and by the fourth year post-initiation, 91% had progressed. In bivariate analysis, there were significant differences in equality of hazards by sex (X²=7.75, p<0.01), from whom participants learnt to inject (X²=22.32, p<0.01) and the drug of injection initiation (X²=18.36; p<0.01). In the multivariable Cox model, only initiating injecting with heroin (HR=1.28; 95% CI: 1.09-1.50) compared with other drugs (predominantly methamphetamine) showed a significantly greater hazard, suggesting a faster progression to regular injecting. Conclusion: This study showed that among our sample of eventual regular injectors, progression from initiation to regular injecting was rapid. By gaining a greater understanding of the dynamics of this progression, the ability to appropriately target interventions and future research is subsequently informed.