A Meta-Analysis of Cannabis Research: Evidence of Lasting Neurocognitive Effects Continues to be Lacking

Objective: There have been widespread increases in cannabis use as recreational and medical use has become legal across the globe. Changes to the chemical makeup of cannabis may have changed residual effects experienced by individuals who use cannabis, and more individuals may be affected.

Method: We conducted a meta-analysis examining the neurocognitive effects of cannabis after abstinence lasting a period of at least 25 days (Prospero: CRD42023466560). Databases used were PsychInfo, PsycArticles, PubMed, and Medline. Previous research has reported mixed results, with some studies finding significant differences between abstinent individuals who use cannabis and matched healthy controls, and others finding no evidence of significant differences. Bias was assessed using Joanna Briggs Critical Appraisal for Analytical Cross-Sectional studies and fail-safe N testing. Random-effects modeling and standardized mean differences were used to assess overall effects on neurocognitive functioning.

Results: Our meta-analysis found no significant effects within the domains of overall effects (ES = 0.17), attention (ES = 0.22), forgetting/retrieval (ES = 0.31), learning (ES =0.29), or verbal/language (ES = 0.41). We identified a significant small effect size within the domain of abstraction/executive function (ES =0.27). However, due to the limited literature in this area, our analysis was under-powered and should be interpreted with caution.

Conclusions: Limited conclusions can be made about the neurocognitive effects of cannabis after abstinence. We describe many deficiencies in existing studies of residual effects of cannabis and provide recommendations for improvement.