Incidence and determinants of initiation into cocaine injection and correlates of frequent cocaine injectors.

PURPOSE

To investigate the incidence and correlates of cocaine injection initiation and the impacts of daily cocaine injection among a cohort of injection drug users.

METHODS

Among 1603 participants, from May 1996 to December 2005, risk factors for initiation of cocaine injection among baseline heroin users were determined by Cox proportional hazards regression and correlates of daily cocaine injection by generalized estimating equations.

FINDINGS

Of the 238 individuals who had never injected cocaine, 200 (84%) had at least one follow-up visit and 121 (61%) consequently initiated into cocaine injection, yielding an incidence density of initiation into cocaine injection of 21.9% (95% confidence interval (CI): 17.9-25.8) per 100 person-years. In a multivariate model, Downtown Eastside (DTES) residence (adjusted hazard ratio (AHR)=2.46, 95% CI: 1.68-3.60), incarceration (AHR=1.50, 95% CI: 1.01-2.24), requiring help injecting (AHR=1.57, 95% CI: 0.99-2.49), and binge drug use (AHR=1.82, 95% CI: 1.22-2.73) remained associated with initiation into cocaine injection. DTES residence (adjusted odds ratio (AOR)=1.99, 95% CI: 1.62-2.46), unstable housing (AOR=1.28, 95% CI: 1.04-1.53), incarceration (AOR=1.29, 95% CI: 1.04-1.60), sex trade involvement (AOR=1.46, 95% CI: 1.15-1.85), requiring help injecting (AOR=2.11, 95% CI: 1.73-2.58)), borrowing syringes (AOR=1.81, 95% CI: 1.35-2.43) and binge drug use (AOR=2.16, 95% CI: 1.81-2.58) were independently associated with daily cocaine injection.

CONCLUSIONS

The baseline prevalence and subsequent incidence of initiation into cocaine injection was high. Daily cocaine injection was independently associated with a number of health and social harms, including elevated HIV risk behavior.