Individual, social, and environmental factors associated with initiating methamphetamine injection: implications for drug use and HIV prevention strategies.

The purpose of this study was to determine the incidence and predictors of initiating methamphetamine injection among a cohort of injection drug users (IDU). We conducted a longitudinal analysis of IDU participating in a prospective study between June 2001 and May 2008 in Vancouver, Canada. IDU who had never reported injecting methamphetamine at the study's commencement were eligible. We used Cox proportional hazards models to identify the predictors of initiating methamphetamine injection. The outcome was time to first report of methamphetamine injection. Time-updated independent variables of interest included sociodemographic characteristics, drug use patterns, and social, economic and environmental factors. Of 1317 eligible individuals, the median age was 39.9 and 522 (39.6%) were female. At the study's conclusion, 200 (15.2%) participants had initiated injecting methamphetamine (incidence density: 4.3 per 100 person-years). In multivariate analysis, age (adjusted hazard ratio [aHR]: 0.96 per year older, 95%CI: 0.95-0.98), female sex (aHR: 0.58, 95%CI: 0.41-0.82), sexual abuse (aHR: 1.63, 95%CI: 1.18-2.23), using drugs in Vancouver's drug scene epicentre (aHR: 2.15 95%CI: 1.49-3.10), homelessness (aHR: 1.43, 95%CI: 1.01-2.04), non-injection crack cocaine use (aHR: 2.06, 95%CI: 1.36-3.14), and non-injection methamphetamine use (aHR: 3.69, 95%CI: 2.03-6.70) were associated with initiating methamphetamine injection. We observed a high incidence of methamphetamine initiation, particularly among young IDU, stimulant users, homeless individuals, and those involved in the city's open drug scene. These data should be useful for the development of a broad set of interventions aimed at reducing initiation into methamphetamine injection among IDU.