Background: Patterns of polydrug use among people who inject drugs (PWID) may be differentially associated with overdose as well as with unique risk factors for HIV transmission.
Objective: Examine patterns of polydrug use among PWID in Tijuana, Baja California, Mexico, and San Diego, California and the relationship of these patterns to HIV risk behavior and overdose.
Methods: These analyses use data from two prospective cohort studies of PWID in San Diego (N=576) (Chapter 2) and Tijuana (N=735) (Chapters 2-4). Chapter 2 assesses prevalence and correlates of methamphetamine and heroin co-injection in a binational sample of PWID from both San Diego and Tijuana using logistic regression. Chapter 3 applies latent class analysis to identify classes of polydrug use and multinomial logistic regression to determine associations with HIV risk behaviors and overdose among PWID in Tijuana. Chapter 4 applies latent transition analysis to describe dynamic statuses of polydrug use at baseline and 6 month follow up, determine probabilities of transitioning between statuses, and examine whether these probabilities are modified by self-report needed for help for drug use.
Results: Chapter 2 findings show that prevalence of heroin and methamphetamine co-injection in the past 6 months was 39.9% overall, was higher in Tijuana (55.8%) than in San Diego (19.8%), and was significantly associated with HIV injection risk behavior in both locations and associated with overdose only in San Diego. Chapter 3 findings show that PWID in Tijuana exhibited 5 distinct patterns of substance use behaviors. Compared to primarily heroin injectors, polydrug and polyroute class membership was associated with HIV injection and sexual risk behaviors. Chapter 4 findings showed that over a third (39.1%) of PWID in Tijuana transitioned to a different subgroup of heroin and methamphetamine polydrug use over 6 months. Those who reported greater need for help for drug use were more likely to transition to subgroups characterized by greater polydrug and polyroute use 6 months later.
Conclusions: Findings highlight the heterogeneity in substance use patterns among PWID in Tijuana and San Diego and demonstrate that polydrug and polyroute users are a high-risk subgroup who may require more tailored prevention and treatment interventions.