Aims
To estimate the prevalence of and risk factors associated with concurrent injection of multiple substances (co‐injection) among a community‐recruited cohort of people who inject drugs.
Design
Cross‐sectional study.
Setting
Melbourne, Australia.
Participants
A sample of 720 actively injecting participants from the Melbourne Injecting Drug User Cohort Study (33% female) was extracted.
Measurements
We constructed two statistical models: a logistic regression model analysing correlates of co‐injection of any substance combination in the past month and a multinomial logistic regression model analysing correlates of three mutually exclusive groups: heroin–diphenhydramine co‐injection only, co‐injection of other substances and no co‐injection. Risk factors examined included drug use characteristics, demographic characteristics, health service use, hepatitis C status, injection risk behaviours and previous experience of non‐fatal overdose.
Findings
One‐third [n = 226, 31%; 95% confidence interval (CI): 28–34%] of participants reported co‐injecting substances within the past month, with equal numbers of participants reporting injecting combinations of heroin–diphenhydramine (n = 121, 54%; 95% CI = 48–60%) and heroin–methamphetamine (n = 121, 54%; 95% CI = 48–60%). In logistic regression analyses, reporting co‐injection of any substance combination was associated with male sex [adjusted odds ratio (aOR) = 1.80, 95% CI = 1.18–2.74, P = 0.006] and injecting daily or more frequently (aOR = 2.04, 95% CI = 1.31–3.18, P = 0.002). In multinomial logistic regression analyses, participants reporting heroin–diphenhydramine co‐injection only were significantly more likely to report groin injecting [adjusted relative risk ratio (aRRR) = 6.16, 95% CI = 2.80–13.56, P < 0.001] and overdose (requiring an ambulance) in the past 12 months (aRRR = 2.81, 95% CI = 1.17–6.72, P = 0.021) compared with participants reporting no co‐injection or co‐injection of other substances.
Conclusions
A substantial proportion of people who inject drugs report co‐injection of multiple substances, which is associated with a range of socio‐demographic, drug use and health service use risk factors.