Project Summary/Abstract This project will use mixed methods (qualitative and quantitative) to evaluate the impact of marijuana policy changes on patterns of use of marijuana and other harmful substances among people living with HIV (PLWH). PLWH are a key population given their high prevalence of marijuana use and potential for marijuana-related adverse outcomes. We will analyze the rich data available through the Centers for AIDS Research Network of Integrated Clinical Systems (CNICS) study, a large, geographically and racially diverse cohort of >32,000 PLWH in care in 8 regions across the U.S. CNICS' comprehensive clinical data includes laboratory data (CD4, HIV RNA levels), medications, diagnoses (substance use disorders, mental health disorders), patient-reported outcomes (PROs) with >70,000 comprehensive substance use assessments, and health care utilization such as primary care visits and missed visits. Ongoing longitudinal substance use assessments combined with geographic diversity including areas like San Diego, Seattle, and Boston with recent regulatory changes and areas like Baltimore and Birmingham without regulatory changes will enable this project. We will examine marijuana use among PLWH using joint longitudinal and survival models to estimate associations with policy/regulation. We will also evaluate the association between marijuana use changes and risk behaviors, such as binge drinking, among PLWH. We will additionally employ qualitative methods including focus groups and structured interviews to evaluate changing usage patterns, beliefs, and health behaviors over periods experiencing regulatory changes. We expect that the majority of increases in marijuana use will be explained by current users increasing frequency rather than non-users becoming users. Furthermore, post-legalization, PLWH reporting current marijuana use will a) describe a diversification of product use and means of ingestion, b) report increased access to marijuana, c) describe an increase in use of marijuana in social settings and in public spaces, d) report marijuana as a cue for use of other substances in certain settings, e) describe decreased stigma associated with marijuana use, and f) and lower rates of alcohol and tobacco use. The findings of this proposal will likely enable several future studies exploring the intersection of marijuana policy, substance use, and health outcomes among PLWH. Further, we expect that data gleaned from this project will have ramifications not only for the clinical care of PLWH but for public health and marijuana legislation in general.