PROJECT SUMMARY Suicide is a leading cause of death in the United States (US). Firearms are the most commonly used and most lethal method of suicide. While firearm access has been consistently and strongly associated with increased risk of firearm suicide, firearm suicides cannot be explained by firearm access alone. Indeed, the vast majority of the estimated 54.7 million firearm owners in the US do not die by firearm suicide. To advance firearm suicide prevention interventions—which are often risk-based rather than universal—we must accurately identify additional and contributing risk factors for firearm suicide. One such factor may be opioid-related harm (e.g., opioid use disorder [OUD], overdose), which has increased exponentially in the US since 1999. OUD has been associated with suicide in the general population. Research on risky opioid use among firearm owners is limited, though substance use is more common among firearm owners than non-firearm-owners and evidence suggests the link between substance use and suicide extends to firearm owners. Further, the impact of the opioid crisis likely extends beyond the direct effects on individuals who use opioids, yet few prior studies have considered opioid-related harm as a contextual factor that may contribute to variations in firearm suicide risk across communities and populations. This study seeks to examine the synergistic impacts of firearm access and opioid-related harm on firearm suicide risk at both the individual and population levels. Our goals are to determine whether personal firearm ownership moderates the impact of individual and community opioid-related harm on individual firearm suicide risk and similarly, whether firearm availability moderates the impact of community opioid-related harm on firearm suicide rates. At the individual level, we will use a case-control design, drawing from over 2.6 million deaths in California from 2008 to 2018, with firearm access defined by known individual legal firearm acquisitions and opioid-related harm defined by prior opioid-related emergency or hospital visits. At the population level, we will use a fixed effects and time series analysis of firearm suicide rates in California from 2005 to 2018. Firearm availability will be defined by a novel and improved measure—the number of legal firearm purchasers per county population across the entire study period—and opioid-related harm will be defined by the lagged rate of nonfatal opioid-related visits and fatal unintentional overdoses. Studying joint impacts will help identify the most at-risk populations with the greatest burden of firearm suicide and further explicate the etiology of firearm suicide. It also offers multiple levers for risk-based intervention; we may need only intervene on one component to prevent the outcome. Results of our project are therefore likely to inform innovative and promising opportunities to enhance safety and prevent self-directed firearm-related injuries and deaths. ...