Abstract
How many people do you know who have tried cannabis? The World Health Organization recently estimated that 145 million people worldwide have. Some of them are probably friends, even family for all I know. Certainly, a significant number of my patients are in that group. Trying cannabis must be considered normal behaviour in many sectors of society and given its illegal status in most of the developed world it is natural that feelings are aroused when the topic of cannabis-related harm comes up in conversation. Misinformation about cannabis abounds, despite an immature but increasingly sound body of research evidence upon which to base opinions. Much of this evidence was tucked away in esoteric journals and was contradictory, although the emergence of high-quality review articles over the past few years have helped clarify some of the key issues. Well, now there is no excuse for uncertainty about the evidence on the nature and impacts of cannabis; read this book and you will be informed in depth about the current scientific knowledge about cannabis, its neurobiology, its effect on psychotic symptoms and depression and what clinicians can do about it.
With 13 chapters each by different experts in the particular topic area making up just over 200 pages, the book is comprehensive and well referenced yet easily consumed. The chapters are uniformly well written and any unevenness in quality is because of unevenness in the scientific research base; there is much more certainty, for example, on the relationship between cannabis and schizophrenia than on that between cannabis and depressive disorder or specific treatment approaches. But the book is certainly a faithful representation of the current state of knowledge due in no small part to the fact that each chapter topic is written by leading researchers and authors in that specific area. It is difficult, therefore, to identify any particular chapters that stand out from the others. Instead, my favourites were those that interested me themost; specifically the chapter on the association between cannabis and depression by Louisa Degenhardt and colleagues which provided a very balanced review of an area in which the certainty of opinion about the evidence tend to exceed the strength of the evidence somewhat, and the chapter on cannabis and psychosis proneness by Hélène Verdoux.
I do have some bees in my bonnet. Clinical implications are given less emphasis than epidemiological and pharmacological aspects. ‘Madness’ and ‘psychosis’ seems subtly assumed to be a unitary phenomenon. Where it is linked to mental disorder, the link seems to be made to schizophrenia with little mention of psychotic mood disorders and very little mention of bipolar psychosis in particular, despite the fact that affective psychoses differ in important ways from non-affective psychoses and make up a significant proportion of clinical presentations with psychotic symptoms. There is little emphasis given to the peculiarities of cannabis use and mental illness in adolescence, despite the first signs of both cannabis use and the onset of major psychotic disorders usually appearing in people of this age group. Finally, when assessing the direction of causality between cannabis use and psychosis or depression, there seems to be an assumption that the onset of the psychotic disorder is the onset of the psychotic symptoms themselves or the fulfilment of DSM-IV criteria for the particular disorder. I think there is insufficient consideration given to the issue of prodromal symptoms or vulnerability phenotypes which precede the clear presence of disorder and which might themselves increase the risk of cannabis use before the particular disorder has fully flourished.
However, these criticisms are just the particular bees that hide under my cap and it would be churlish of me to suggest they detract from the value of this book. Although there might be areas of weakness in this book, this is because they are the areas of weakness in the research base. This book provides an excellent and unparalleled overview of current evidence-based thinking about cannabis and psychotic disorders, and should be read by anyone involved in clinical practice or research with people who suffer serious mental illness and also use cannabis. And let's face it that is probably most of us.