Understanding polysubstance use at the daily and event levels: protocol for a mixed-methods qualitative and ecological momentary assessment study in a community-based sample of people who use illicit drugs in Oakland, California, USA.

introductionPolysubstance use is extremely common among people who use illicit opioids in the USA. It is associated with poor substance use treatment outcomes, infectious disease risk and alarming rates of drug overdose. Nearly all extant literature examines polysubstance use over broad time frames, such as 30 days or 6 months. However, both substance use and overdose risk are episodic. To build a stronger understanding of polysubstance use and overdose risk, we need to expand the knowledge base to include daily-level and event-level data that examine how substances are used together, in which combinations and in which contexts. The study described in this protocol will use qualitative and ecological momentary assessment (EMA) methods to examine polysubstance use and overdose risk on a daily and event level. METHODS AND ANALYSIS: This is a mixed-methods observational study with three phases. The first phase is formative, consisting of qualitative interviews with people who use multiple substances (N=20), to inform the development of items for the EMA component. The second phase is EMA data collection with people who use multiple substances (N=120), three times daily for 28 days. The third phase consists of mixed-methods inquiries with a subset of participants (N=20), using participant-level EMA data and qualitative techniques to build a nuanced understanding of the motivations and contexts of polysubstance use in everyday life. Analytical induction methods will be used to interpret qualitative data. Hierarchical linear modelling methods will be used to analyse EMA data. ETHICS AND DISSEMINATION: This research has been reviewed and approved by the Institutional Review Board at RTI International (#MOD00001782 for EMA procedures and #MOD00001241 for qualitative procedures). Participants engage in an informed consent procedure for each component of the study. Data will be managed and shared per the National Institutes of Health extramural data sharing policy.