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Is frequent or heavy cannabis use associated with cognitive dysfunction in adolescents and young adults?
This systematic review and meta-analysis of 69 cross-sectional studies of 2152 cannabis users and 6575 comparison participants showed a small but significant overall effect size for reduced cognitive functioning in adolescents and young adults who reported frequent cannabis use. However, studies requiring abstinence from cannabis for longer than 72 hours had a very small, nonsignificant effect size.
Although continued cannabis use may be associated with small reductions in cognitive functioning, results suggest that cognitive deficits are substantially diminished with abstinence.
Substantial shifts in perception and policy regarding cannabis have recently occurred, with use of cannabis increasing while its perceived harm decreases. One possible risk of increased cannabis use is poorer cognitive functioning, especially in youth.
To provide the first quantitative synthesis of the literature examining cannabis and cognitive functioning in adolescents and young adults (with a mean age of 26 years and younger).
PubMed, PsycInfo, Academic Search Premier, Scopus, and bibliographies of relevant reviews were searched for peer-reviewed, English-language studies from the date the databases began through May 2017.
Consensus criteria were used to determine study inclusion through abstract and manuscript review.
Data Extraction and Synthesis
This study followed Meta-analysis of Observational Studies in Epidemiology guidelines. Effect size estimates were calculated using multivariate mixed-effects models for cognitive functioning outcomes classified into 10 domains.
Main Outcomes and Measures
Results from neurocognitive tests administered in cross-sectional studies were primary outcomes, and we examined the influence of a priori explanatory variables on variability in effect size.
Sixty-nine studies of 2152 cannabis users (mean [SD] age, 20.6 [2.8] years; 1472 [68.4%] male) and 6575 comparison participants with minimal cannabis exposure were included (mean [SD] age, 20.8 [3.4]; 3669 [55.8%] male). Results indicated a small overall effect size (presented as mean d) for reduced cognitive functioning associated with frequent or heavy cannabis use (d, −0.25; 95% CI, −0.32 to −0.17; P < .001). The magnitude of effect sizes did not vary by sample age or age at cannabis use onset. However, studies requiring an abstinence period longer than 72 hours (15 studies; n = 928) had an overall effect size (d, −0.08; 95% CI, −0.22 to 0.07) that was not significantly different from 0 and smaller than studies with less stringent abstinence criteria (54 studies; n = 7799; d, −0.30; 95% CI, −0.37 to −0.22; P = .01).
Conclusions and Relevance
Associations between cannabis use and cognitive functioning in cross-sectional studies of adolescents and young adults are small and may be of questionable clinical importance for most individuals. Furthermore, abstinence of longer than 72 hours diminishes cognitive deficits associated with cannabis use. Although other outcomes (eg, psychosis) were not examined in the included studies, results indicate that previous studies of cannabis in youth may have overstated the magnitude and persistence of cognitive deficits associated with use. Reported deficits may reflect residual effects from acute use or withdrawal. Future studies should examine individual differences in susceptibility to cannabis-associated cognitive dysfunction.
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Corresponding Author: J. Cobb Scott, PhD, Hospital of the University of Pennsylvania, 3400 Spruce St, 10th Floor, Gates Bldg, Philadelphia, PA 19104 (firstname.lastname@example.org).
Accepted for Publication: January 30, 2018.
Published Online: April 18, 2018. doi:10.1001/jamapsychiatry.2018.0335
Author Contributions: Dr Scott had full access to all of the data in the study and takes responsibility for the integrity of the data and the accuracy of the data analysis.
Study concept and design: Scott, Slomiak, Jones, Rosen, Gur.
Acquisition, analysis, or interpretation of data: All authors.
Drafting of the manuscript: Scott, Slomiak, Jones, Moore, Gur.
Critical revision of the manuscript for important intellectual content: Scott, Jones, Rosen, Moore, Gur.
Statistical analysis: Scott, Slomiak, Rosen, Moore.
Obtained funding: Scott.
Administrative, technical, or material support: Scott, Slomiak, Jones, Rosen, Moore.
Study supervision: Scott, Moore, Gur.
Conflict of Interest Disclosures: Dr Gur reports receiving royalties from the Brain Resource Center. No other disclosures were reported.
Funding/Support: This work was supported by a Department of Veterans Affairs Career Development Award (grant IK2CX000772, Dr Scott).
Role of the Funder/Sponsor: The funders were not involved in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication.
Disclaimer: The views expressed in this article are those of the authors and do not necessarily reflect the position or policy of the Department of Veterans Affairs or the University of Pennsylvania.
Additional Contributions: We thank Carrie Cuttler, PhD, Washington State University, Alecia Dager, PhD, Olin Neuropsychiatry Research Center, Sarah Feldstein Ewing, PhD, Oregon Health & Science University, Pilar Flores Cubos, PhD, Universidad de Almeria, Jodi Gilman, PhD, Harvard Medical School, Boris Quednow, Dr rer nat, University of Zurich, and Liam Nestor, PhD, Imperial College London, for kindly providing supplementary data from their studies for analyses. They were not compensated for their contributions.
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