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ABSTRACT: Cannabis is the third most used psychoactive substance worldwide. The legal status of cannabis is changing in many Western countries, while we have very limited knowledge of the public health impact of cannabis-related harms. There is a need for a summary of the evidence of harms and risks attributed to cannabis use, in order to inform the definition of cannabis risky use. We have conducted a systematic review of systematic reviews, aiming to define cannabis-related harms. We included systematic reviews published until July 2018 from six different databases and following the PRISMA guidelines. To assess study quality we applied the AMSTAR 2 tool. A total of 44 systematic reviews, including 1,053 different studies, were eligible for inclusion. Harm was categorized in three dimensions: mental health, somatic harm and physical injury (including mortality). Evidence shows a clear association between cannabis use and psychosis, affective disorders, anxiety, sleep disorders, cognitive failures, respiratory adverse events, cancer, cardiovascular outcomes, and gastrointestinal disorders. Moreover, cannabis use is a risk factor for motor vehicle collision, suicidal behavior and partner and child violence. Cannabis use is a risk factor for several medical conditions and negative social consequences. There is still little data on the dose-dependency of these effects; evidence that is essential in order to define, from a public health perspective, what can be considered risky use of cannabis. This definition should be based on quantitative and qualitative criteria that informs and permits the evaluation of current approaches to a regulated cannabis market. (PsycINFO Database Record (c) 2020 APA, all rights reserved)

Broad synthesis / Overview of systematic reviews

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Journal Journal of dual diagnosis
Year 2020
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Objective: As the perceived risk of cannabis use continues to decline among youths and access continues to increase, it has become more important to synthesize the rapidly growing literature on the effects of cannabis on neurocognition. Hundreds of studies examining associations between cannabis use and neurocognitive functioning have been published in recent decades. However, results often differ across individual studies, particularly when sample sizes are small. Meta-analytic methods help to make sense of this literature and have been increasingly applied to studies on cannabis use and neurocognition. Methods: A systematic literature search using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines was conducted to identify peer-reviewed meta-analyses of neurocognitive or functional neuroimaging data that examined associations between cannabis use and non-acute effects on neurocognitive functioning (n = 8). Results: Current findings suggest that regular healthy cannabis users, regardless of age, display poorer neurocognitive functioning relative to nonusers of small to medium effect sizes across many neurocognitive domains, as well as functional brain alterations when compared to non-users. Adverse effects are not uniform across neurocognitive domains and evidence for adolescent-onset users having poorer neurocognitive outcomes remains equivocal based on these studies. However, less is known about cannabis effects on neurocognition among clinical samples, as findings from specific clinical samples revealed mixed results. Conclusions: Meta-analyses have played an important role in helping to grasp the totality of results from a large body of literature on cannabis effects on neurocognition, yet more research (particularly large-scale longitudinal studies) is needed to identify critical periods or patterns of use that are more likely to result in negative outcomes.

Broad synthesis

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Report HPRA
Year 2017

Broad synthesis

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Authors Barnes, MP , Barnes, JC
Report Group for Drug Policy Reform
Year 2016

Broad synthesis

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Authors Madras, Bertha K
Report Report to the WHO Expert Committee on Drug Dependence
Year 2015

Broad synthesis / Overview of systematic reviews

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Journal Drug and alcohol review
Year 2010
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ISSUES: Cross-sectional surveys have revealed that cannabis is the most widely used illicit substance in Western countries. Cannabis intoxication can lead to acute, transient psychotic symptoms and the short-term exacerbation of pre-existing psychotic symptoms. However, controversy exists about whether cannabis can actually cause long-term psychosis. APPROACH: We summarised the findings of systematic reviews on the association between cannabis use and psychosis, searching MEDLINE, EMBASE and CINAHL up to August 2007. We assessed the methodological quality, selected the better quality reviews and analysed reasons for discordant results. KEY FINDINGS: We included five systematic reviews. Four of the reviews performed a meta-analysis and showed a consistent association between cannabis use and psychosis; the fifth review considered psychological problems more broadly, did not perform a meta-analysis and reported an inconsistent association. The reasons for discordance were: different outcomes (psychosis vs. psychological problems), different inclusion criteria for primary studies and different methods for summarising the results. IMPLICATIONS: This overview shows a consistent association between cannabis use and psychotic symptoms, though it is not possible to draw firm conclusions about a causal relationship. Reverse causality and residual confounding cannot be excluded. An interaction with other environmental and genetic factors is difficult to ascertain. CONCLUSION: We conclude that there is insufficient knowledge to determine the level of risk associated with cannabis use in relation to psychotic symptoms and that more information is needed on both the risks of cannabis use and the benefits of preventive interventions to support evidence-based approaches in this area.