Effect of Washington State and Colorado's cannabis legalization on death by suicides

In the U.S., death by suicide is a leading cause of death and was the 2nd leading cause of death for ages 15-to-34 in 2018. Though incomplete, much of the scientific literature has found associations between cannabis use and death by suicide. Several states and the District of Columbia have legalized cannabis for general adult use. We sought to evaluate whether cannabis legalization has impacted suicide rates in Washington State and Colorado, two early adopters. We used a quasi-experimental research design with annual, state-level deaths by suicide to evaluate the legalization of cannabis in Washington State and Colorado. We used synthetic control models to construct policy counterfactuals as our primary method of estimating the effect of legalization, stratified by age, gender, and race/ethnicity. Overall death by suicide rates were not impacted in either state. However, when stratified by age categories, deaths by suicide increased 17.9% among 15-24-year-olds in Washington State, or an additional 2.13 deaths per 100,000 population (p-value ≤0.001). Other age groups did not show similar associations. An ad hoc analysis revealed, when divided into legal and illegal consumption age, 15-20-year olds had an increase in death by suicides of 21.2% (p-value = 0.026) and 21-24-year olds had an increase in death by suicides of 18.6% (p-value ≤0.001) in Washington State. The effect of legalized cannabis on deaths by suicide appears to be heterogeneous. Deaths by suicide among 15-24-year-olds saw significant increases post-implementation in Washington State but not in Colorado.