Table 2

Studies examining cannabis use levels in adolescents before and after the legalisation of cannabis use for medical purposes (1996–2018)

StudyPlace of studyType of policy change/study periodParticipant characteristics/nStudy designCannabis use measure (frequency/period)Statistical methods/covariatesKey findings/effect sizeRisk of bias (/10)(rater 1 (R1); rater 2 (R2))
Khatapoush and Hallfors29California (CA).State-level medical marijuana laws (MML): Proposition 215 (1996).16–25 year olds participating in the Robert Wood Johnson Foundation’s Fighting Back initiative/n=2651.Repeated cross-sectional surveys in 1995, 1997 and 1999.12-month and 30-day use.Logistic regression model.12-month use: d=0.54, 95% CI 0.48 to 0.59;
30-month use: d=0.72, 95% CI 0.64 to 0.79 (unadjusted).
R1: 8.6.
R2: 8.0.
Average: 8.3.
Probable.
Gorman and 
Charles Huber30
Los Angeles, San Diego, San Jose (CA) and Portland (OR).State-level MMLs introduced before 2002.10–18 year olds registered in the Arrestee Drug Abuse Monitoring system/on average n=255 in California and n=81 in Oregon.Routine data collection (1995–2002).Urine test data (≥50 ng of THC - tetrahydracannabinniol per decilitre).Interrupted time series design.California: 10–18 years: d=−0.42, 95% CI −0.60 to −0.24; Oregon: 10–18 years: d=−0.75, 95% CI −1.33 to −0.17 (unadjusted).R1: 9.2.
R2: 9.1.
Average: 9.2.
Low.
Wall et al31Alaska, California, Colorado, Hawaii, Maine,
Nevada, Oregon, Washington+Michigan, Montana, New Mexico, Rhode Island, Vermont+Arizona, Delaware and New Jersey versus other US states.
State-level MMLs introduced before 2002.12–17 olds participating in the National Survey of Drug Use and Health (NSDUH)/n~11 813.Repeated cross-sectional surveys (2002–2008).30-day use.Fixed effects model with random intercept, controlled for cannabis use 2002–2008.2002–2003: d=0.15, 95% CI 0.07 to 0.23.R1: 9.7.
R2: 9.4.
Average: 9.6.
Very low.
Harper et al32Alaska, California, Colorado, Hawaii, Maine,
Nevada, Oregon, Washington+Michigan, Montana, New Mexico, Rhode Island and Vermont versus other US states.
State-level MMLs introduced before 2002.12–17 year olds and 18–25 year olds participating in the NSDUH
/n~11 813 per age group.
Repeated cross-sectional surveys (2002–2009).30-day use.Difference-in-differences estimates controlled for measurement error.12–17 years: d=−1.25, 95% CI –1.29 to −1.21; 18–25 years: d=1.71, 95% CI 1.67 to 1.75.R1: 9.4.
R2: 9.5.
Average: 9.5.
Very low.
Anderson et al33Alaska, Arizona, California, Colorado, Delaware, District of Columbia, Hawaii, Maine,
Michigan, Montana, Nevada, New Jersey, New Mexico, Oregon, Rhode Island, Vermont and Washington versus other US states.
State-level MMLs introduced before 2011.15–19 years olds participating in the National and State Youth Risk Behavior Surveys (YRBS)
/n=786 568.
Repeated cross-sectional surveys (1993–2011).30-day use; 30-day frequent use.Linear regression controlled for age, sex, race, grade, state-level marijuana decriminalisation, Blood Alcohol Content (BAC) 0.08 laws, state beer tax, income per capita and unemployment.YRBS: 30-day use: d=−0.32, 95% CI −0.33 to  −0.32; 30-day frequent use d=−0.18, 95% CI −0.19 to  −0.18.R1: 8.5.
R2: 8.1.
Average: 8.3.
Possible.
Lynne-Landsman et al34Delaware,
Michigan, Montana, Rhode Island compared pre-MML and post-MML implementation.
State-level MMLs introduced 2003–2009.9–-12th graders participating in the YRBS
/n~11 453.
Repeated cross-sectional surveys (2003–2009).30-day use.Difference-in-differences controlled for age, ethnicity and sex.d=0.24, 95% CI 0.20 to 0.28.R1: 9.2.
R2: 10.
Average: 9.6.
Very low.
Choo et al35Idaho versus Montana, Massachusetts versus Rhode Island, New Hampshire versus Maine, Utah versus Nevada and New York versus Vermont.State-level MMLs.9–12th graders participating in the YRBS/n~11 703 100.Repeated cross-sectional surveys since 1991, 1993, 1995, 1997, 1999, 2001, 2003, 2005 and 2007.30-day use.Difference-in-differences controlled for state, year, age, sex, race and grade.d=0.065, 95% CI 0.026 to 0.105.R1: 10.
R2: 9.5.
Average: 9.8.
Very low.
Wen et al36District of Columbia, Michigan, Montana, New Jersey, New Mexico, Rhode Island and Vermont versus US states with no MML laws by 2010.State-level MMLs introduced between 2004 and 2010.12–20 year olds participating in the NSDUH/n~183 600.Repeated cross-sectional surveys (2004–2011).30-day use.Fixed-effects models controlled for age, sex, race, self-reported health, cigarette use, urban residence, family poverty, state-level unemployment, mean income, median income and alcohol excise taxes.d=−1.72, 95% CI −1.73 to −1.71.R1: 9.6.
R2: 10.
Average: 9.8.
Very low.
Hasin et al3821 US states that passed MML versus non-MML US states.State-level MMLs introduced by 2014.8th, 10th and 12 grade (13, 15 and 17) students participating in the Monitoring the Future surveys (MTF)
/n=1 098 070.
Repeated cross-sectional surveys (1991–2014).30 day use.Multilevel regression models controlled for sex, age, race, parental education, class size, private school, urban setting, state-level proportion of males, whites, persons with no high school education and aged 11–24 years.d=−0.033, 95% CI −0.039 to −0.026.R1: 10.
R2: 10.
Average: 10.
Very low.
Schuermeyer et al39Colorado versus 34 non-MML US states.State-level MMLs introduced in 2009.12–20 year olds participating in the NSDUH/n~158 600 12–17 year olds+159 200 18–25 year olds.Repeated cross-sectional surveys (2006–2011).≥20 times in 30-day use.Logistic regression controlled for state, year, age, sex, race, educational level and state-by-year interaction.12–17 years: d=0.09, 95% CI −0.04 to 0.23; 18–25 years: 0.23, 95% CI 0.16 to 0.29 (unadjusted).R1: 9.
R2: 8.2.
Average: 8.6.
Possible.
Pacula et al40Alaska, Arizona, California, Colorado, Delaware, District of Columbia, Hawaii, Maine, Maryland,
Michigan, Montana, Nevada, New Jersey, New Mexico, Oregon, Rhode Island, Vermont and Washington versus US states with no MML in 2012.
State-level MMLs introduced before 2012; comparison of different legal provisions: patient registry; home cultivation; an legal dispensaries.-<21 year olds participating in the National Longitudinal Survey of Youths 1997 (NLSY)/46 375.Cohort study (1997–2011).30-day use.Difference-in-differences models controlled for population unemployment rate, age distribution, state beer tax rate, BAC 0.08 tax;d=−0.0109, 95% CI −0.03 to 0.009.R1: 8.2.
R2: 8.6.
Average: 8.4.
Possible.
Keyes et al4121 US states that passed MML versus non-MML US states.State-level MMLs introduced by 2014.8th, 10th and 12th grade (13, 15 and 17) students participating in the MTF
/n=973 089.
Repeated cross-sectional surveys (1991–2014).30-day use.Time-varying multilevel regression models controlled for sex, age, race, parental education, class size, private school, urban setting, state-level proportion of males, whites, persons with no high school education and aged 11–24 yearsd=−0.03, 95% CI −0.03 to −0.027.R1: 9.6.
R2: 9.5.
Average: 9.6.
Very low.
Maxwell and Mendelson42California, Colorado and Washington versus other US States.State-level MMLs and recreational marijuana laws.12–25 year olds participating in the NSDUH.Repeated cross-sectional surveys (2002–2013).12-month use.Comparison of prevalence ratesImpossible to calculate.R1: 5
R2: 6.8
Average: 5.9
Probable
Stolzenberg et al43Alaska, Arizona, California, Colorado, Delaware, Hawaii, Maine,
Michigan, Montana, Nevada, New Jersey, New Mexico, Oregon, Rhode Island, Vermont and Washington versus other US states.
State-level MMLs introduced between 1998 and 2011.12–17 olds participating in the NSDUH
/n~112 500.
Repeated cross-sectional surveys (2002–2011).30-day use.Random effects model controlled for state-level medical cannabis possession limit, cannabis availability, % enrolled in drug class, alcohol use, prior crime conviction, % families on income assistance, % juveniles who skipped school, % families where the father resides in household, % male and % white.d=0.060, 95% CI 0.034 to 0.087.R1: 7.3.
R2: 8.1.
Average: 7.7.
Possible.
Wall et al44Arizona, Delaware,
Michigan, Montana, New Jersey, New Mexico, Rhode Island and Vermont versus other US states.
State-level MMLs introduced between 2004 and 2011.12–17 olds participating in the NSDUH
/n~112 500.
Repeated cross-sectional surveys
(2002–2011).
30-day use.Fixed effects model.d=−0.0059, 95% CI −0.030 to 0.018.R1: 9.4.
R2: 10.
Average: 9.7.
Very low.
Martins et al45Arizona, Connecticut, Delaware, Illinois,
Massachusetts,
Michigan, New Hampshire, New Jersey, New Mexico and Rhode Island versus other US states.
State-level MMLs introduced between 2005 and 2013.12–17 and 18–25 year olds participating in the NSDUH/n=175 000 12–17 year olds and 175 000 18–25 year olds.Repeated cross-sectional surveys (2004–2013).30-day use.Multilevel logistic regression controlled for sex, ethnicity, insurance status, household income, population density, state proportions of sex, ethnicity, youths, education, unemployment and median household income.12–17 years: d=0.02, 95% CI −0.04 to 0.08;
18–25 years: d=0.006, 95% CI −0.035 to 0.047).
R1: 9.1.
R2: 10.
Average: 9.6.
Very low.
Johnson et al49Alaska, Arizona, Colorado, Delaware, Maine, Michigan, Montana, Nevada, New Jersey, New Mexico, Rhode Island and Vermont versus other US states.State-level MMLs introduced between 1998 and 2011.9th–12th graders (14-17) participating in the YRBS
/n=715 014.
Repeated cross-sectional surveys (1991–2011).30-day use and 30-day heavy use (≥20 times).Fixed-effect multiple logistic regression controlled for year, state, age, sex and ethnicity.30-day use: d=−0.042, 95% CI −0.051 to −0.032; 30-day heavy use: d=0.0001, 95% CI −0.018 to 0.0185.R1: 9.5.
R2: 9.5.
Average: 9.5.
Very low.
Borodovsky et al53Alaska, Colorado, District of Columbia, Oregon and Washington versus 20 US states that did not.Legalisation status: MML or Recreational cannabis laws (RCCL) versus no legal cannabis law.14–18 year olds recruited online (via targeted Facebook invitations)
/n=2630.
Cross-sectional
survey (29 April–18 May 2016).
30-day cannabis use via smoking, vaporising or eating.Logistic and linear regression controlled for age, gender, race, grade level, lifetime days of cannabis use and age of onset.d=0.064, 95% CI −0.048 to 0.176.R1: 9.5.
R2: 9.1.
Average: 9.3.
Low.
Mauro et al14Arizona, Connecticut, Delaware, Illinois, Maine, Michigan, New Hampshire, New Jersey, New Mexico and Rhode Island versus 27 US states with no MML by 2013.State-level MMLs introduced prior to 2015.12–17 and 18–25 year olds participating in the NSDUH
/n~175 000 12–17 year-olds and 175 000 18–25 year year olds.
Repeated cross-sectional surveys (2004–2013).State-level 30-day and daily use.Multilevel linear regression controlled for trends in marijuana use and state-level % males, % white, % aged 10–24 years, %>25 years with no high school degree, % unemployed and median household income.30-day use: 12–17 years: d=0.041, 95% CI 0.022 to  0.059;
18–25 years: d=0.016, 95% CI 0.003 to 0.029; daily use: 12–17 years: d=−0.003, 95% CI −0.021 to  0.014;
18–25 years: d=0.064, 95% CI 0.050 to 0.078.
R1: 10.
R2: 9.5.
Average: 9.8.
Very low.
Cerdá et al10Arizona, California, Colorado, Connecticut, Delaware, Illinois, Maine, Maryland, Massachusetts, Michigan, Minnesota, Montana, Nevada, New Hampshire, New Jersey, New Mexico, New York, Oregon, Rhode Island, Vermont and Washington versus other US states.State-level MMLs introduced prior to 2015; time since legalisation.8th, 10th and 12th graders (13, 15 and 17) participating in the MTF/n=1 140 768.Repeated cross-sectional survey (1991–2015).30-day use.Difference-in-differences models controlled for individual grade, age, sex, race/ethnicity, socioeconomic status, number of students per grade, public versus private school, school in metropolitan statistical area, state-level % males, % white, % aged 10–24 years and % older than 25 with no high school degree.d=0.0176, 95% CI 0.0170 to 0.0182.R1: 10.
R2: 10.
Average: 10.
Very low.