Please use this link to access this publication. Abstract Background and Aims Ecological studies have suggested that Cannabis legalization might have led to a decrease in opioid overdose deaths. Such studies do not provide information about whether individuals are substituting Cannabis for opioids at different points in time. The current study assessed the magnitude of the daily association between Cannabis and opioid use in individual adults with and without pain who use non-medical opioids. Design Prospective cohort study. Setting The greater New York area and a suburban inpatient addiction program. Participants Adults with problem substance use who use non-medical opioids, recruited from May 2016–June...