Studies examining cannabis use levels in adolescents before and after legalisation of recreational cannabis use (2016–2018)
| Study | Place of study | Type of policy change/study period | Participant characteristics/n | Study design | Cannabis use measure (frequency/period) | Statistical methods/covariates | Key findings | Risk of bias (/10)(rater 1 (R1); rater 2 (R2)) |
| Estoup et al46 | Washington. | Legalisation in 2012. | High school students (14–17) with problematic substance use enrolled in high school in the Seattle area/n=262. | Cohort study (2010–2015). | 3-month use. | Mediation model. | d=0.061, 95% CI −0.110 to 0.232 (unadjusted). | R1: 7.1. R2: 7.4. Average: 7.3. Possible. |
| Mason et al47 | Washington versus other US states. | Legalisation in 2012. | 8th graders (14) in Tacoma, Washington participating in a longitudinal study/n=238. | Cohort study (2010/2011–2012/2013). | 30-day use. | Multilevel regression models controlled for substance use initiation prior to baseline. | d=0.323, 95% CI −0.177 to 0.825. | R1: 9.2. R2: 9.1. Average: 9.2. Low. |
| Fleming et al48 | Washington versus other US states. | Legalisation in 2012. | 10th (16) graders participating in the biennial Washington state school survey/n=30 365. | Repeated cross-sectional surveys (2000–2014). | 30-day use. | Logistic regression analyses controlled for perceived harm of marijuana, alcohol use and year. | d=0.044, 95% CI −0.019 to 0.069. | R1: 9.2. R2: 9.2. Average: 9.2. Low. |
| Cerdá et al50 | Colorado and Washington versus other US states. | Legalisation in 2012. | 13–18 years participating in the Monitoring the Future surveys (MTF)/n=253 902. | Repeated cross-sectional surveys (2010–2015). | 30-day use. | Difference-in-differences. | d=1.03, 95% CI 1.00 to 1.06. | R1: 9.1. R2: 10. Average: 9.6. Very low. |
| Kerr et al51 | Two universities in Oregon versus six in other US states. | Legalisation in 2015. | 18–26 year old college undergraduates participating in the Healthy Minds Study/n=10 924. | Repeated cross-sectional surveys (2014 and 2016). | 30-day use. | Mixed-effects logistic regression controlled for cigarette use, year in college, age, sex, race, residential type, relationship status, sexual orientation, international student status, depression, anxiety, adjustment, institution size and survey period. | d=0.0139, 95% CI 0.048 to 0.075. | R1: 9.5. R2: 8.5. Average: 9. Low. |
| Miller et al52 | Washington. | Legalisation in 2012; opening of licenced retail stores for marijuana in 2014. | College students participating in the National College Health Assessment (WSU NCHA)/n=13 335. | Repeated cross-sectional surveys (2005, 2006, 2008, 2010, 2012, 2014 and 2015). | 30-day use. | Logistic regression controlled for age, sex, race, year in school and estimated secular increase in cannabis use. | Postmedical marijuana laws (post-MML) (2014): d=0.04, 95% CI 0.002 to 0.083; postrecreational marijuana laws (2015): d=0.082, 95% CI 0.034 to 0.130. | R1: 9.5. R2: 8.5. Average: 9. Low. |
| Harpin et al64 | Colorado. | Legalisation in 2012. | 6–12th grade (11–17) students (Healthy Kids Colorado Survey)/n=24 171. | Repeated cross-sectional survey (2013 and 2014). | 30-day use. | Comparison of prevalence rates. | d=0.006, 95% CI −0.026 to 0.038 (unadjusted). | R1: 9.4. R2: 9.4. Low. |
| Jones et al55 | Colorado. | Legalisation in 2012. | College students(22–24 years) n=1413. | Repeated cross-sectional survey (October 2013, March 2014, October 2014 and March 2015). | Lifetime. | Comparison of prevalence rates. | d=−0.215, 95% CI −0.385 to −0.039 (unadjusted). | R1: 4.4. R2: 5.0. Average: 4.7. High. |