Statement of purpose Over the last 12 years, 32 states have relaxed restrictions under which members of the public may use lethal force in self-defence’”commonly called Stand Your Ground laws. While enacted to increase the potential cost of predatory violence, these laws pose important questions regarding maintenance of safety and the legitimacy of the criminal justice system. We examined the impact of the first Stand Your Ground law since its enactment in Florida in 2005.

Methods We collected monthly counts of homicide from the Wide-ranging Online Data for Epidemiologic Research (WONDER) web portal of the Centres for Disease Control and Prevention (CDC) and of Justifiable Homicide from the Florida Department of Law Enforcement (FDLA) during 1999–2015. Using seasonally adjusted segmented Poisson regression models, we examined whether the onset of the law was associated with any significant changes to monthly rates of either illegal or justifiable homicide.

Results After accounting for underlying trends, we estimate that onset of the law was associated with an abrupt and sustained increase in the monthly homicide rate of 24.4% (relative risk [RR]=1.24; 95% CI=1.16–1.33). While fewer in number, we estimated a significant increase in justifiable homicides of 75% (RR=1.75; 95%CI=1.24–2.48). Justifiable homicides accounted for 3.4% of all homicides on average in the period 1999’”2005, but rose to 8.7% of all monthly homicides between 2005 and 2015.

Conclusion Ten years since the enactment of Florida’s Stand Your Ground law there has been a dramatic and sustained increase in homicide, as well as a substantial increase in homicides deemed ‘justifiable’ by the criminal justice system. Significance These findings suggest laws weakening the limitations on the use of lethal force in self-defence may have serious implications for public safety as well as for the way in which homicide is processed by the criminal justice system.