Co-use of cannabis and alcohol before and after Canada legalized nonmedical cannabis: A repeat cross-sectional study.

Background: This study examined changes in population-level co-use of cannabis and alcohol before and 12 months after nonmedical cannabis legalization in Canada, relative to the United States that had previously legalized and not legalized (US legal and illegal states, respectively).
Methods: Data are from waves 1 and 2 of the International Cannabis Policy Study, collected in 2018 (before) and 2019 (12 months after legalization in Canada). Respondents aged 16-65 years from Canada ( n  = 25,313) and US legal ( n  = 25,189) and US illegal ( n  = 19,626) states completed an online survey. Changes in co-use between 2018 and 2019 in US legal and illegal states compared to those in Canada were assessed using multinomial logistic regression.
Results: Descriptive analyses show increases in cannabis use and monthly or more frequent (MMF) co-use between 2018 and 2019 in all jurisdictions. Compared to no MMF use of cannabis or alcohol, there was no evidence suggesting differences in changes in MMF co-use in US legal or illegal states relative to Canada. However, respondents in US legal states had 33% higher odds of MMF alcohol-only use (OR = 1.33, 99% CI: 1.12, 1.57) compared to no MMF use relative to Canada.
Conclusions: Increases in co-use were observed between 2018 and 2019 in all jurisdictions regardless of the legal status of cannabis. These shifts were largely due to increases in cannabis use across the population, including those that use alcohol, and may indicate changing societal norms toward cannabis generally. As the cannabis legalization transition in Canada matures, evaluation over the longer term will improve understanding of the influence of cannabis liberalization on co-use.