<strong>Aims:</strong> To describe the methods and baseline characteristics of a cohort of people who tamper with pharmaceutical opioids, formed to examine changes in opioid use following introduction of Reformulated OxyContin<sup>®</sup>.<p></p> <p><strong>Methods:</strong> Participants were 606 people from three Australian jurisdictions who reported past month injecting, snorting, chewing or smoking of a pharmaceutical opioid and had engaged in these practices at least monthly in the past 6 months. Baseline interviews were conducted prior to introduction of Reformulated OxyContin<sup>®</sup> in April 2014. Patterns of opioid use and cohort characteristics were examined according to whether participants were prescribed opioid medications, or exclusively used diverted medication./p> </p><p><strong>Results:</strong> TThe cohort reported high levels of moderate/severe depression (61%), moderate/severe anxiety (43%), post-traumatic stress disorder (42%), chronic pain or disability (past 6 months, 54%) and pain (past month, 47%). Lifetime use of oxycodone, morphine, opioid substitution medications and codeine were common. Three-quarters (77%) reported ICD-10 lifetime pharmaceutical opioid dependence and 40% current heroin dependence. Thirteen percent reported past year overdose, and 70% reported at least one past month opioid injection-related injury or disease. The cohort displayed complex clinical profiles, but participants currently receiving opioid substitution therapy who were also prescribed other opioids particularly reported a wide range of risk behaviors, despite their health service engagement.</p> <p><strong>Conclusions:</strong> Findings highlight the heterogeneity in the patterns and clinical correlates of opioid use among people who tamper with pharmaceutical opioids. Targeted health interventions are essential to reduce the associated harms.</p>