Table 1

Studies examining cannabis use levels in adolescents before and after decriminalisation of cannabis possession (1981–2017)

StudyPlace of studyType of policy change/study periodParticipant characteristics/nStudy designCannabis use measures (frequency/period)Statistical methods/covariatesKey findings/effect size(Rater 1 (R1); rater 2 (R2))
(/10)
Johnston et al25California, Maine, Minnesota, Ohio (early change)+North Carolina, New York and Missouri (late change) versus states with no decriminalisation.Decriminalisation of cannabis possession in 1975 and 1976.12th grade students participating in the Monitoring the Future surveys (MTFS)/n~99 000 per year.Cross-sectional survey (1975–1980).12 months,
30 days and daily use.
Comparisons of prevalence rates.12-month use: d=0.06, 95% CI 0.0003 to 0.127; 30-day use: d=0.125, 95% CI 0.059 to 0.191 (unadjusted).R1: 8.
R2: 8.1.
Average:
8.1. Possible.
Thies and Register26Alaska, California, Colorado, Maine, Minnesota, Missouri, Nebraska, New York, North Carolina, Ohio and Oregon.Decriminalisation of cannabis possession 1976–1979.14–21 year olds participating in the National Longitudinal Study of Youths 1979 (NLSY)
/n=12 686.
Cohort study with follow-up in 1984 and 1988.30-day use.Ordinary Least Squares (OLS) regression models controlled for sex, race, parental education, own education, income, church attendance, marital status, urban setting; legal context regarding underage drinking.1984: d=0.05, 95% CI −0.02 to 0.14;
1988: d=0.06, 95% CI −0.03 to 0.16 (unadjusted).
R1: 9.
R2: 9.
Average: 9.
Low.
McGeorge and Aitken56Australian Capital Territory versus Melbourne.Decriminalisation of cannabis possession, use and cultivation in 1992.Third-year university students at Australian National University and Melbourne University
/n=903.
Cross-sectional survey (1994).12-month use.χ2 statistics.d=0.21, 95%–0.02 to 0.45 (unadjusted).R1: 6.4.
R2: 4.5.
Average: 5.5. Probable.
MacCoun and Reuter61The Netherlands.Decriminalisation of cannabis in 1976 followed by legalisation in 1984.16–18 year olds participating in a school-based survey (Trimbos) versus youths of the same age in the USA (Monitoring the Future)/n~115 000.Repeated cross-sectional surveys: 1970–1996.Lifetime use.Comparison of prevalence rates.The Netherlands versus USA: d=−0.23; 95% CI −0.26 to −0.19 (unadjusted).R1: 8.1.
R2: 8.2.
Average: 8.2. Possible.
Pacula27Alaska, California, Colorado, Maine, Minnesota, Missouri, Nebraska, New York, North Carolina, Ohio and Oregon.Decriminalisation of cannabis possession 1976–1979.14–21 year olds participating in the NLSY/n=8008.Cohort study with follow-up in 1984.30-day use.Two part model controlled for sex, age, race, number of siblings, urban setting, academic achievement, expected years of schooling, illegal activity; parents’ marital status, employment status, alcohol use; legal context regarding alcohol use, crime level, beer and cigarette taxes.d=0.0179, 95% CI −0.038 to 0.0008.R1: 10.0.
R2: 8.1.
Average: 9.1.
Low.
DiNardo and Lemieux28California, Colorado, Maine, Minnesota, Mississippi, Nebraska, New York, North Carolina, Ohio and Oregon.Decriminalisation of cannabis possession 1976–1979.12th grade students participating in the MTF
/n~165 000.
Cross-sectional survey (1980–1989).30-day use.Structural regression model controlled for sex, age, race, parental education, weekly hours of work, income, alcohol use, state-level unemployment and alcohol drinking age.d=−0.08, 95% CI −0.12 to −0.05.R1: 7.7.
R2: 7.7.
Average: 7.7. Possible.
Williams57Australian Capital Territory and Northern Territory versus non decriminalising states.Decriminalisation of cannabis possession, use and cultivation between 1988 and 1998.14–25 years, Australian National Drug Strategy Household Survey (NDSHS)
/n=15 468.
Repeated cross-sectional surveys (1988, 1991, 1993, 1995 and 1998).12-month use.Ordered probit model controlled for sex, age, marital status, dependent children, ethnicity, educational level, employment, capital city residence and decriminalisation regime.d=0.04, 95% CI −0.102 to 0.192.R1: 9.2.
R2: 10.
Average: 9.6.
Very low.
Williams and Bretteville-Jensen58South Australia, Australian Capital Territory, Northern Territory and Western Australia versus non-decriminalising states.Decriminalisation of cannabis possession, use and cultivation between 1987 and 2004.20–40 year old lifetime users of cannabis participating in the Australian NDSHS
/n=39 087.
Repeated cross-sectional surveys (1998, 2001, 2004, 2007 and 2010).Age at initiation.Difference-in-differences with discrete time hazard model controlled for sex, education, ethnicity, capital city residence and survey year.12–17 years: d=0.57, 95% CI 0.52 to 0.63.R1: 10.
R2: 10.
Average: 10.
Very low.
Fuller59England.Declassification of cannabis from schedule B to schedule C drug (2004) and then back (2009).11–15 year olds participating in a representative school survey/n=6173.Repeated cross-sectional surveys (2001–2014).12-month use.Prevalence rates.2004 versus 2009: d=−0.22, 95% CI −0.29 to −0.165 (unadjusted).R1: 6.4.
R2: 6.4.
Average: 6.4. Probable.
Braakmann and
Jones60
UK.Declassification of cannabis from schedule B to schedule C drug (2004).15–25 year olds participating in the Offending, Crime and Justice Survey/n=2539.Cohort study (2003–2006).12-month and 30-day use.Difference-in-differences model controlled for age and calendar year.12-month use; 15–17 years: d=−0.01, 95% CI −0.07 to 0.002; 18–25 years: d=−0.05, 95% CI −0.15 to 0.05; 30 day use: 15–17 years: −0.006, 95% CI −0.009 to  0.08; 18–20 years: −0.13, 95% CI −0.24 to −0.02.R1: 8.6.
R2: 8.0.
Average: 8.3.
Possible.
Miech et al37California versus other US states.Decriminalisation in 2010.8th, 10th and 12th grade students participating in the MTF/n=97 238.Repeated cross-sectional surveys (2007–2012).12-month and 30-day use.Generalized Estimated Equation (GEE) regression models.12-month use: d=0.32, 95% CI 0.31 to 0.34; 30-day use: d=0.57, 95% CI 0.55 to 0.59 (unadjusted).R1: 8.6.
R2: 9.0.
Average: 8.8.
Possible.
Shi et al63Cross-national study of 38 countries.Depenalisation, decriminalisation, and partial prohibition changed since 0–5 years, 5–10 years or >10 years.15 year olds participating in the Health Behaviour in School-Aged Children Study/n=1 72 894.Repeated cross-sectional surveys (2001/2002, 2005/2006 and 2009/2010).12-month and regular (≥40 times in lifetime) use.Multilevel logistic random intercept regression.12-month use: d=−0.004, 95% CI −0.02 to 0.13; regular use: d=0.17, 95% CI 0.13 to 0.20.R1: 8.2.
R2: 8.0.
Average: 8.1.
Possible.
Červený et al62The Czech Republic.Decriminalisation of cannabis possession in 2010.15–25 years participating in drug use monitoring surveys/n=1086 in 2008 and 438 in 2012.Repeated cross-sectional surveys
(2008–2012).
Age at initiation.Mixed proportional hazards controlled for sex, education, birth cohort and region of residence.d=−0.26, 95% CI −0.37 to −0.15 (unadjusted).R1: 10.
R2: 10.
Average: 10.
Very low.