BACKGROUND AND OBJECTIVES: Individuals with posttraumatic stress disorder (PTSD) have an elevated risk of cannabis use disorder. However, the effect of cannabis legalization on use among individuals with PTSD is unclear. We evaluated changes in cannabis involvement in emergency department (ED) visits for PTSD after medical and nonmedical legalization in Ontario, Canada. METHODS: This repeated cross-sectional study used health administrative data to identify all ED visits for PTSD among Ontario residents aged 10-105 between 2008 and 2022 (n = 15.7 million). We identified PTSD ED visits with a co-diagnosis of cannabis (main exposure) or alcohol (control condition) and examined changes across four policy periods (medical legalization with restrictions, expanded medical legalization, nonmedical legalization with restrictions, and nonmedical commercial expansion) using Poisson models. RESULTS: Among 381,450 PTSD ED visits, 4593 (1.29%) co-involved cannabis and 11,625 (3.05%) co-involved alcohol. Rates of cannabis involvement in PTSD ED visits increased by 151% (Incidence Rate Ratio [IRR]: 2.51
95% CI: 2.24, 2.82) between the first and last policy periods (0.13 vs. 0.33 per 100,000 individuals), while alcohol-involvement increased by 58% (IRR: 1.58
95% CI: 1.47, 1.68). Cannabis involvement in PTSD ED visits increased steadily over the study period, with no significant association between policy periods and this trend. DISCUSSION AND CONCLUSIONS: Cannabis involvement in PTSD ED visits has increased over time during a period of liberalization of cannabis policy, but may be attributed to increased access and normalization rather than policy changes directly. SCIENTIFIC SIGNIFICANCE: Findings underscore the need for improved detection of and targeted interventions for disordered cannabis use among individuals with PTSD in regions with legalized cannabis.