The transition from adolescence to adulthood is characterized by increased rates of prescription opioid use and related problems. These rates may be even higher among emerging adults who use cannabis. National surveys show cannabis use is rising in young adults and is related to increased opioid use and opioid use disorder. However, there is a substantial gap in our knowledge of what individual risk factors might contribute to the development of opioid and cannabis co-use and related problems, and we know little about co-use in the context of pain. Identifying malleable factors that convey risk for problematic opioid use in the context of cannabis co-use would inform the development of preventive interventions that could be targeted to modify substance use risk for emerging adults in medical settings. Emerging adults receive the lowest level of preventative medical care when compared to all other developmental stages. Thus, a single medical encounter in which opioid medications are prescribed to manage acute pain offers a unique window into substance use patterns, and a unique opportunity to develop and deliver future interventions. The proposed study will utilize a developmental model of the impact of opioid exposure by legitimate prescription during emerging adulthood, with consideration for existing substance use, as well as pain and psychosocial experiences of the individual. This prospective cohort study will enroll emerging adults (ages 18-25) who either have no lifetime history of cannabis (5% of sample) or have used cannabis in the past month (95% of sample) and are prescribed an opioid for acute pain management in the course of routine outpatient healthcare. These participants will be followed closely during the 2-weeks post-prescription, and then prospectively for 2 years, with multimethod assessments to capture short- and long-term trajectories of opioid and cannabis co-use in the context of pain experiences. The central hypothesis is that pain experiences as well as patterns of opioid and cannabis use in the acute pain period will influence opioid use, cannabis use, and related problems over time. We will also examine the contributory roles of psychosocial and pain-related risk and protective factors, including the use of non-pharmacologic pain management strategies and prescription characteristics. Objective data on dispensed opioid medications will be obtained from medical records and prescription drug monitoring databases, and bioassay data will verify self-report of substance use. Daily associations between pain experiences, pain catastrophizing, negative and positive affect, use of non-pharmacologic pain management strategies, cannabis use, and opioid use will also be examined. Importantly, many of these risk and protective factors can be impacted with biobehavioral treatments. Determining which malleable risk and protective factors are most important during this developmental period will provide critical information for the desi...