Changes in Medical Cannabis Use After Recreational Cannabis Legalization in Canada.

As part of its recreational cannabis legalization in October 2018, Canada imposed an excise tax of 10% (or $1 a gram, whichever is higher) on both recreational and medical cannabis. There is little evidence to inform the ongoing debate on whether the legalization had adverse impacts on medical cannabis use. We used an interrupted time series design and data on medical cannabis shipments (i.e., mail-order deliveries of cannabis from a licensed producer to a patient authorized to obtain medical cannabis) in Canada between quarter 1 of 2014 and quarter 1 of 2020. We examined changes in medical cannabis shipments after Canada's recreational cannabis legalization both across Canada and for each province. As this study used publicly available, province-level aggregate data, ethics approval was not required. Recreational cannabis legalization was associated with significant reductions in medical cannabis use in 7 out of 10 Canadian provinces. Compared with the counterfactual estimated from prelegalization trends, the reduction in quarter 1 of 2020 varied from 500 shipments per 100,000 population (95% CI=312-688 shipments per 100,000 population) or 32% (95% CI=22-43%) in Newfoundland and Labrador to 3,778 shipments per 100,000 population (95% CI=2,972-4,585 shipments per 100,000 population) or 74% (95% CI=68-79%) in Alberta. At the national level, the number of medical cannabis shipments decreased by 823 per 100,000 population (95% CI=725-921 shipments per 100,000 population) or 48% (95% CI=45-52%). Recreational cannabis legalization was associated with reductions in medical cannabis use. Our findings call for policy attention to address possible adverse impacts of recreational cannabis legalization on medical cannabis users.