Abstract Aims The aims of this study were to determine the prevalence of methadone syrup injecting in Adelaide, South Australia and to characterize methadone injectors, including their heroin use and risk behaviours associated with heroin overdose. Design Cross-sectional design. Setting Community setting, principally metropolitan Adelaide. Participants Current heroin users (used heroin in the last 6 months), recruited through snowballing. Measurements Structured questionnaire. Findings Of 365 participants, 18.4% reported having ever injected methadone syrup and 11.0% had injected methadone in the last 6 months. Those that had injected methadone were more likely to be male, and were more likely to be receiving methadone maintenance. They were also maintained on higher doses of methadone than subjects not injecting methadone. A history of methadone injection was associated with more heroin overdose experiences and greater dependence on heroin. Methadone injectors were also more likely to engage in risky behaviours associated with heroin overdose, including using heroin when no other people were present, not trial-tasting new batches of heroin and polydrug use. Conclusions Methadone syrup injectors appear to be at greater risk of a series of harms than subjects not injecting methadone. The prevalence of methadone syrup injecting in Adelaide, South Australia was 11%, which was lower than prevalence in Sydney, New South Wales, but higher than in Melbourne, Victoria. Jurisdictional differences concerning the prevalence of methadone syrup injecting may reflect differing policies by each state to methadone dispensing.