The pattern of drug use in Vietnam has changed rapidly over the past decade, and a large number of people who have a history of heroin injection reportedly use methamphetamine. This paper describes factors associated with methamphetamine use among people who inject heroin in Hanoi, the capital of Vietnam. This is a cross-sectional survey among 521 heroin injectors who were recruited through chain referral and outreach at community and clinic settings. Eligibility criteria included: (1) male aged 18 or older; (2) reported heroin injecting during the 12 months before the survey; (3) agreed for a urine test to detect methamphetamine and opiate metabolites. The primary outcome, methamphetamine use, was defined as self-reported methamphetamine use during the 30 days before the survey and/or having a urine test positive for methamphetamine. Structural Equation Model was used to evaluate associated factors for methamphetamine use. One third of participants qualified as methamphetamine users as defined in this study. A longer history of heroin use (β=0.126, p<0.001), using MDMA and/or cannabis (β=0.28, p<0.001) and not using condom during sex (β=0.139, p<0.001) were positively associated with methamphetamine use. Family functioning (β=-0.141; p<0.001) was protective. The goodness-of-fit of Structural Equation Model was excellent (CFI=0.934; TLI=0.912; RMSEA=0.033; WRMR=0.98). Methamphetamine use among people who inject heroin is a substantial issue in Hanoi. Family functioning has made a critical contribution on reducing methamphetamine use. Future studies should pay attention to address the role of factors at the family level in addition to individual-level factors towards the pattern of drug use.
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