Socioeconomic stability is associated with lower injection frequency among people with distinct trajectories of injection drug use

Background: Little is known about how socioeconomic circumstances relate to injection frequencies among people who inject drugs (PWID) with diverse trajectories of injection. We aimed to characterize trajectories of injection drug use in a community-based sample of PWID over 7.5 years and to investigate the extent to which two modifiable factors reflecting socioeconomic stability -stable housing and stable income -relate to injection frequencies across distinct trajectories. Methods: HEPCO is an open, prospective cohort study of PWID living in Montreal with repeated follow-up at three-month or one-year intervals. Self-reported data on injection frequency, housing and income are collected at each visit. Injection frequency was defined as the number of injection days (0-30), reported for each of the past three months. Using group-based trajectory modeling, we first estimated average trajectories of injection frequency. Then, we estimated the trajectory group-specific average shift upward or downward associated with periods of stable housing and stable income relative to periods when these conditions were unstable. Results: Based on 19,527 injection frequency observations accrued by 529 participants followed over 2011-2019 (18.3% female, median age: 41), we identified five trajectories of injection frequency: three characterized by sustained injection at different frequencies (28% infrequent; 19% fluctuating; 19% frequent), one by a gradual decline (12%), and another by cessation (28%). Periods of stable housing and stable income were each independently associated with a lower injection frequency, on average, in all five trajectory groups (2.2-7.5 fewer injection days/month, depending on the factor and trajectory group). Conclusion: Trajectories of injection drug use frequency were diverse and long-lasting for many PWID. Despite this diversity, socioeconomic stability was consistently associated with a lower injection frequency, emphasizing the close relationship between access to fundamental necessities and injection patterns in all PWID, irrespective of whether they are on a path to cessation or sustained injecting.