BACKGROUND: In October 2018, Canada legalized recreational cannabis, with Ontario distributing retailer licenses through a lottery system in 2019. This study investigates the impact of recreational cannabis retailer allocation on emergency department (ED) visits related to cannabis, alcohol, and opioids. METHOD: A longitudinal study of 278 communities in Ontario (proxied by Forward Sortation Areas, FSAs) was conducted using health administrative data from ICES for all Ontario residents covered by public health insurance. The cohort included 11,156,100 adults aged 18 and above, monitored quarterly from January 2016 to March 2023. The allocation of cannabis retailers through a randomized lottery system provided a natural experiment. Staggered difference-in-differences proposed by Callaway and Sant'Anna (CSDID) models, weighted by the inverse probability of retailer allocation, were used to estimate the impact of cannabis store openings on ED visits, comparing FSAs with and without retailers. RESULTS: No significant effects were found in cannabis-, alcohol-, or opioid-related ED visits following the allocation of cannabis retailers. Sensitivity analyses, including alternate diagnostic codes, co-use of cannabis and other substances, and cannabis use without other substances, corroborated our main findings. The null results may be due to online cannabis sales preceding retail store openings, geographic distribution minimizing access disparities, and potential spillover effects. CONCLUSION: The allocation of recreational cannabis retailer licenses did not significantly impact acute care use. Continuous monitoring, comprehensive sales tracking, and integrated substance use prevention strategies are recommended for future policy considerations.