Assessing the Impact of Cannabis Legalization on Adolescent Cannabis Use and Cannabis-Related Health Conditions - PROJECT SUMMARY/ABSTRACT Adolescent cannabis use is increasingly recognized as an important public health problem with significant adverse health consequences. Recreational cannabis legalization (RCL) has high potential to increase risk for adolescent cannabis use and cannabis-related health conditions. However, the effects of RCL on adolescents are unknown. With a highly representative sample of nearly 1 million adolescents (N=900,000) regularly screened for cannabis use in Kaiser Permanente Northern California’s large integrated healthcare delivery system from 2015 to 2023, this multi-method study will address the pressing public health questions of whether and how California state RCL and heterogeneity of local regulatory schemes impact the risk of adolescent cannabis use and cannabis-related health conditions. We will use a quasi-experimental interrupted time series design to test whether adolescent cannabis use, cannabis-related health conditions (e.g., cannabis use disorder, other substance use, respiratory symptoms, mental health problems, cannabis-related accidents, acute cannabis intoxication, cannabinoid hyperemesis syndrome), and cannabis-related healthcare use (e.g., emergency department, addiction treatment) and rise post-RCL. We will also examine local RCL policies (retailer bans or caps, product and advertising restrictions, taxes, health warnings) and use geographic information systems methods to map retailer location (e.g., distance from home address to the closest retailer) onto individual-level electronic health record data collected in clinical care to test whether the effects of RCL are stronger among those who live in communities with less restrictive local policies, adjusting for pre-RCL medical cannabis policies. We will conduct hypothesis-driven moderation analyses to identify sub-groups whose risk for cannabis use and cannabis-related health conditions may be greatest following RCL. To gain further clinical insight into our quantitative data and guide next steps, we will conduct focus groups with adolescents and interviews with addiction medicine clinicians, pediatricians, and emergency department physicians to gain insight into how RCL impacts adolescent cannabis use and addiction treatment need from the perspective of both patients and providers. Together, results will have immediate public health and clinical implications, providing urgently needed data to identify potentially actionable areas for future policy adaptations and aid decision making for public health practices to reduce adolescent cannabis use and cannabis-related harms.