Polysubstance use (including alcohol) is extremely common in cases of opioid overdose.  A stronger understanding of the patterns, contexts and motivations for polysubstance use can inform efforts to prevent overdose and other negative health outcomes.  Existing research on polysubstance use relies predominantly on past 30-day timeframes.  This three-year study will use mixed methods to take an in-depth look at SAME-DAY polysubstance use, the timeframe most relevant to overdose risk. In addition, there are few prospective studies of polysubstance use - most are cross-sectional.  It is not known whether or how polysubstance use  patterns change over time, and what factors influence changes.  The study will use the "drug, set and setting" framework to examine combinations of substances used (drug), motivations for their use (set) and social/structural factors influencing use (setting).