Residual effects of cannabis use on neurocognitive performance after prolonged abstinence: a meta-analysis.

Cannabis is the most widely used illicit drug in the U.S., and the number of illicit and licit users is rising. Lasting neurocognitive changes or deficits as a result of use are frequently noted despite a lack of clarity in the scientific literature. In an effort to resolve inconsistencies in the evidence of lasting residual effects of cannabis use, we conducted two meta-analyses. First, we updated a previous meta-analysis on broad nonacute cognitive effects of cannabis use through inclusion of newer studies. In a second meta-analysis, we focused on evidence for lasting residual effects by including only studies that tested users after at least 25 days of abstinence. In the first meta-analysis, 33 studies met inclusion criteria. Results indicated a small negative effect for global neurocognitive performance as well for most cognitive domains assessed. Unfortunately, methodological limitations of these studies prevented the exclusion of withdrawal symptoms as an explanation for observed effects. In the second meta-analysis, 13 of the original 33 studies met inclusion criteria. Results indicated no significant effect of cannabis use on global neurocognitive performance or any effect on the eight assessed domains. Overall, these meta-analyses demonstrate that any negative residual effects on neurocognitive performance attributable to either cannabis residue or withdrawal symptoms are limited to the first 25 days of abstinence. Furthermore, there was no evidence for enduring negative effects of cannabis use.