Does liberalisation of cannabis policy influence levels of use in adolescents and young adults? A systematic review and meta-analysis

Objectives To examine the effectof cannabispolicy liberalisation (decriminalisation and legalisation) levels of use in adolescents and young adults. Design Systematic review and meta-analysis. Inclusion criteria Included studies were conducted among individuals younger than 25 years and quantitatively assessing consequences of cannabispolicy change. We excluded articles: (A) exclusively based on participants older than 25 years; (B) only reporting changes in perceptions of cannabisuse; (C) not including at least two measures of cannabisuse; (D) not including quantitative data; and (E) reviews, letters, opinions and policy papers. PubMed, PsycINFO, Embase and Web of Science were searched through 1 March 2018. Data extraction and synthesis Two independent readers reviewed the eligibility of titles and abstracts and read eligible articles, and four authors assessed the risk of bias (Quality Assessment Tool for Observational Cohort and Cross-Sectional Studies). Extracted data were meta-analysed. The protocol was registered with PROSPERO. Results 3438 records were identified via search terms and four via citation lists; 2312 were retained after removal of duplicates, 99 were assessed for eligibility and 41 were included in our systematic review. 13 articles examined cannabisdecriminalisation, 20 examined legalisation for medical purposes and 8 examined legalisation for recreational purposes. Findings regarding the consequences of cannabisdecriminalisation or legalisation for medical purposes were too heterogeneous to be meta-analysed. Our systematic review and meta-analysis suggest a small increase in cannabisuse among adolescents and young adults following legalisation of cannabisfor recreational purposes (standardised mean difference of 0.03, 95% CI −0.01 to –0.07). Nevertheless, studies characterised by a very low/low risk of bias showed no evidence of changes in cannabisuse following policy modifications. Conclusions Cannabispolicy liberalisation does not appear to result in significant changes in youths’ use, with the possible exception of legalisation for recreational purposes that requires monitoring. Trial registration number CRD42018083950.