“Evidence on the Acute and Residual Neurocognitive Effects of Cannabis Use in Adolescents and Adults: a Systematic Meta-Review of Meta-Analyses”, 2022-01-19 (; similar):
Background: Cannabis is among the most consumed psychoactive substances world-wide. Considering changing policy trends regarding the substance, it is crucial to understand more clearly its potential acute and residual adverse effects from a public health viewpoint. Cognitive function is one of the targeted areas with conflicting findings. This meta-review measured the magnitude of acute and residual effects of cannabis on cognition in adolescents and adults provided by meta-analyses and evaluated quality of evidence.
Method: A systematic search was performed in PubMed, PsycINFO, Web of Science and Google Scholar. Meta-analyses were included if they quantitatively examined the performances of users from the general population on cognitive tasks.
Results: The search retrieved 10 eligible meta-analyses (71 effects sizes, n = 43 761) with evidence ranging from low to moderate quality, which were categorized into domains of cognitive functions: executive functions (k = 7), learning and memory (k = 5), attention (k = 4), processing speed (k = 5), perceptual motor function (k = 2) and language (k = 2).
Verbal learning and memory displayed the most robust evidence and were most impaired by acute cannabis intoxication that persisted after intoxication passed. Small-to-moderate acute and residual adverse effects were reported for executive functioning. Cannabis use led to small deficits in inhibitory processes and flexibility, whereas small-to-moderate deficits were reported for working memory and decision-making. Evidence regarding processing speed and attention has shown that cannabis administration induced small-to-moderate adverse effects and residual neurocognitive deficits were observed in heavy cannabis-using youths. Results showed no statistically-significant difference between cannabis users and non-users on language, and small-to-moderate effects for simple motor skills.
Conclusion: Meta-analytical data on the acute effects of cannabis use on neurocognitive function have shown that cannabis intoxication leads to small to moderate deficits in several cognitive domains. These acute impairments accord with documented residual effects, suggesting that the detrimental effects of cannabis persist beyond acute intake.
…Despite the findings provided in this meta-review, several elements need to be discussed when interpreting results. First and foremost, the meta-analyses discussed comprised cross-sectional data with several analyses having relatively small sample sizes, which limits the inference of a causal relationship between cannabis use and cognition as well as the generalizability of results…Although most of the evidence on the cognitive sequelae of cannabis use has been provided by cross-sectional data associated with methodological limitations, a growing number of longitudinal studies, which are useful to address causal inferences, have emerged. This has led to several reviews examining, among others, evidence provided by prospective designed studies[20, 27, 31, 81]. For instance, Bourque et al27 noted similar findings to those observed in cross-sectional data. Indeed, most studies showed declines in both executive functioning and verbal learning/memory[82–95], while results were less consistent for processing speed[82, 85, 88, 90, 94–96]. Furthermore, longitudinal data have similarly shed light on the hypotheses that have been put forth to explain the association between cannabis use and cognitive functions (see Bourque et al27 for an overview). A first hypothesis, that has received mixed evidence, specifies that cannabis use leads to persistent cognitive impairments. These neurotoxic effects last although cannabis users reduce their intake or quit altogether. While some longitudinal studies suggest that cognitive deficits resolve following abstinence[92, 94], other studies have confirmed that cannabis use frequency led to subsequent long-term cognitive decline (ie. executive function) regardless of prolonged cannabis intake, while adjusting for covariates[84, 87, 97]. Following, the premorbid cognitive vulnerability hypothesis proposes that individuals at increased risk of using the substance more regularly already presented cognitive deficits before cannabis use onset. Several studies have shown that specific cognitive impairments (ie. memory and executive functions) seemed to incline individuals to earlier onset of use in addition to more frequent use in comparison to non-using individuals[83–86, 98]. However, such findings were not evident in all studies[82, 87, 97, 99, 100] and some studies more probably support the common antecedent hypothesis[86, 98], which postulates that common factors (eg. externalizing behavior) may predispose individuals to both cannabis use and cognitive deficits in users. Hence, results from longitudinal co-twin studies have suggested that cannabis use may not necessarily cause neurocognitive decline, but rather that factors related to family background, such as genetic and shared environmental factors, may more clearly explain worse cognitive performances amid cannabis users[86, 98].