Adolescent cannabis use disorder following recreational cannabis legalization in California, USA: A population‐based interrupted time series study

AIMS: To examine whether and how rates of adolescent cannabis use disorder (CUD) in California, USA, changed after recreational cannabis legalization (RCL). DESIGN AND SETTING: Population-based time-series study of adolescents aged 13 to 17 years with Kaiser Permanente Northern California healthcare system membership from January 2014 to February 2020. PARTICIPANTS: The sample (n = 637 461) was 66.45% non-White, 50.93% male, with a mean (standard deviation) age of 14.07 (1.43) years. MEASUREMENTS: Exposure variables included California state RCL passage (9 November 2016), which legalized possession, home grows and reduced penalties for underage possession; and implementation of legal recreational sales (1 January 2018). Outcome was the count of incident CUD diagnoses in each month, extracted from the electronic health record. Interrupted time series analyses were fit using Poisson regression. RESULTS: In the best fitting model, prior to RCL passage, CUD rates were decreasing [adjusted rate ratio (aRR) = 0.95 per year, 95% confidence interval (CI) = 0.92-0.97]. Rates increased at the time of RCL passage + 6-month lag (adjusted slope change aRR = 1.26, 95% CI = 1.14-1.38). At RCL implementation + 6 months, there was a statistically significant negative slope change (adjusted slope change aRR = 0.86, 95% CI = 0.76-0.98). CUD incidence continued to increase, but was no longer statistically significant (adjusted trend aRR = 1.02, 95% CI = 0.96-1.09). SUMMARY: Among adolescents in Northern California, USA, trends in incident cannabis use disorder rates appeared to increase following recreational cannabis legalization passage, with increasing rates persisting through retail implementation and three years post-passage. Results underscore the importance of monitoring cannabis use disorder, in addition to measures of cannabis use prevalence and frequency, when evaluating the public health implications of cannabis legalization.