Cannabis and Psychosis Put in Perspective

In this issue, Louisa Degenhardt and Wayne Hall provide an insightful and accessible overview of the literature on the relation between cannabis use and psychosis. They also present some of the policy implications of this relation. The first paper reviews the evidence for a causal relation between cannabis use during adolescence and early adulthood and subsequent diagnosis of a psychotic disorder or experience of psychotic symptoms (1). The authors present research that has established an association between cannabis use and psychosis that cannot be explained by chance. They then focus on recent longitudinal studies that have investigated the causal direction of the association and that have attempted to rule out other explanations for the association by controlling for potential confounders, such as other drug use and personal characteristics. Degenhardt and Hall conclude that these studies suggest that among vulnerable individuals cannabis use does predict an increased risk of experiencing psychotic symptoms or of developing a psychotic disorder. Self-medication is rejected as a likely explanation for the relation. Given the evidence of the involvement of the cannabinoid system in psychosis, Degenhardt and Hall also argue that the causal relation between cannabis and psychosis is biologically plausible. The second paper places this evidence in a broader context by comparing it with the evidence for the health effects of other drug use (2). In light of the evidence presented, the authors discuss the implications for public policy. They conclude that young people with mental health problems should be discouraged from using cannabis and that all young people should be informed of the mental health risks of cannabis use. Hall and Degenhardt examine the controversial topic of how evidence for harms, in this case the risk of developing psychosis as a result of cannabis use, should affect the laws and penalties associated with cannabis. The authors argue that, while we should ensure that legislation and penalties do not make it easier for young people to begin using cannabis, there are many other issues, such as the social harm associated with personal cannabis use being a criminal offence, to consider with respect to cannabis laws and penalties. It is clear from the research literature that, if an individual is prone to serious psychotic illness, the use of cannabis may trigger an episode. The extent of the problem in terms of psychiatric cases must be clarified and put into perspective. Internationally, cannabis use is increasingly prevalent, and it now attracts a great amount of attention from the public, from families of users, from concerned policy-makers, and from mental health clinicians who are increasingly aware of cannabis use among their patients. One of the consequences of focusing on cannabis and psychosis is that other harms associated with cannabis use are overshadowed. The lack of recognition of the mental health effects associated with other drug use (such as alcohol, cocaine, opioids, and amphetamines) is another consequence. Hall and Degenhardt clearly point to the much larger problem of amphetamine psychosis presentations, despite the comparatively fewer users of amphetamine (2). Degenhardt and Hall discuss important cannabis-related harms that include the prevalence of cannabis dependence in the community, the social harms associated with cannabis use, cognitive dysfunction, educational and vocational failure, and the large amount of money spent on cannabis by its regular users. A myopic view will focus on cannabis’ relation to psychosis and will miss the larger picture of the effects of cannabis on general health and well-being. The recent development of an Australian National Cannabis Strategy (3) demonstrates the need for public education on the harms associated with cannabis use; however, the public and professional communities need a broader understanding, from credible sources, of the range of harms that cannabis use brings and not a single focus on psychosis symptoms and disorders. When read carefully, Degenhardt and Hall’s contributions can create an understanding of the strength of evidence and provide some credible ways to deal with the continued growth of cannabis use in many Western countries. Their call for the

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