PROJECT SUMMARY We propose an Exploratory/Developmental Research Grant (R21) in response to “Public Policy Effects, on Alcohol-, Marijuana-, and Other Substance-Related Behaviors and Outcomes” (PA-17-132) which calls for studies of local implementation of state marijuana and opioid policies and their impacts on substance use outcomes. Across the US, there is a trend toward liberalizing marijuana policy and addressing the opioid crisis. The effects of cannabis legalization on health outcomes are not clear and preliminary evidence suggests this may be due in part to local heterogeneity. Our work aims to develop a classification of local substance use policies in California. California provides an ideal context for this study because its laws regarding cannabis grant enormous latitude to localities to tax, limit, or ban retail and commercial businesses; unlike in other states, there is no cap on local cannabis tax rates. Further, California is one of 16 states authorizing syringe exchange programs (SEPs); as a result over 20 counties and cities have passed, opposed, and passed and rescinded SEPs. We propose a developmental R21 study that will map heterogeneity in the implementation of cannabis policies in 12 California counties, controlling for opiate policies, and then explore associations with health outcomes related to cannabis and prescription opioid use. We hypothesize that: (a) local substance use policies vary significantly and can be meaningfully classified by the degree to which they comprehensively regulate cannabis and opiates; and (b) that more comprehensive local policies are associated with reduced marijuana and prescription opioid use. Aim 1 will map and classify local substance use policies by scope. We will collect written policy data and conduct key informant interviews with policymakers in 12 California counties. We will then classify local policies to determine whether they are more or less comprehensive, for both cannabis and opioids. Aim 2 will analyze the association between local policies and cannabis outcomes including: 1) retail outlets (both storefront and online) identified through Weedmaps; 2) retail marijuana sales from the California Board of Equalization; and 3) past-month cannabis use and modes of use from the California Health Interview Survey. Aim 3 will explore the relationship between local policies and prescription opioid use, using data from the California Controlled Substance Utilization Review and Evaluation System. Studying local substance use policy within the large and diverse state of California will build understanding of how local policies may influence broader trends in drug use.