Adolescent Cannabis Use After Cannabis Legalization and the COVID-19 Pandemic

Importance: Understanding how cannabis legalization and large-scale societal disruptions influence adolescent cannabis use is important, as early use is associated with adverse health and educational outcomes. California has the largest regulated cannabis market in the US and provides a unique context to study how major policy and societal shifts are associated with adolescent cannabis use. Objective: To examine how rates of adolescent cannabis use in California changed after recreational cannabis legalization (RCL) and the COVID-19 pandemic. Design, Setting, and Participants: This population-based time-series cross-sectional study included adolescents aged 13 to 17 years in a large, integrated health care delivery system in California who were universally screened for cannabis use during standard pediatric care from January 2011 to December 2024. Data were analyzed from March to September 2025. Exposures: California state RCL passage (November 9, 2016), which reduced penalties for underage possession of cannabis; implementation of legal sales of cannabis for adults (January 1, 2018); and the COVID-19 pandemic (March 19, 2020). Main Outcomes and Measures: Any cannabis use in the past-year, self-reported based on universal screening at standard pediatric well-child visits. Interrupted time series analyses were fit using Poisson regression, standardized for sociodemographic characteristics of the adolescent Kaiser Permanente Northern California population. Results: The study included 1 308 655 screening questionnaires completed over the 14-year study period (mean [SD] 7789 [3673] adolescent screenings per month), with a mean (SD) age of 15.0 (1.4) years at screening and 666 881 (51.0%) questionnaires from female participants. Before RCL passage, the mean rate of adolescent cannabis use was decreasing, from 10.39% (95% CI, 9.96%-10.82%) in January 2011 to 6.83% (95% CI, 6.54%-7.14%) in October 2016 (annual trend rate ratio [RR], 0.93 [95% CI, 0.92-0.94] per year). Rates started increasing at the time of RCL passage (slope change RR, 1.19 [95% CI, 1.06-1.35]), and rates continued to increase after implementation of legal retail sales (annual trend RR, 1.06 [95% CI, 1.02-1.11] per year). After the COVID-19 pandemic began, rates of adolescent cannabis use decreased substantially (level change RR, 0.86 [95% CI, 0.76-0.97]; slope change RR, 0.80 [95% CI, 0.70-0.91]). Rates of adolescent cannabis use continued to decrease slightly in the years after COVID-19, reaching a mean of 6.45% (95% CI, 6.15%-6.77%) in December 2024 (annual trend RR, 0.98 [95% CI, 0.96-0.997] per year). Conclusions and Relevance: In this large, retrospective cross-sectional study, adolescent cannabis use increased following RCL and decreased following the COVID-19 pandemic, returning to prelegalization rates by 2024. These results have important implications for clinicians and policymakers and could help estimate future changes in adolescent cannabis use in response to broad policy changes and societal shifts and inform more effective prevention strategies.