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Study Claims: Cannabis ‘doubles risk of psychotic episodes’


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A new study has claimed that cannabis-use increases the risk of developing psychotic symptoms.

The 10 year longitudinal study published in the British Medical Journal has provoked widespread media comment, though certain facts have been ‘glossed over’. As ever, with such a sensitive subject, the media appear keen to focus on the sensationalistic side of the findings, without conscientiously examining the facts behind the study.

As far back as the 19th century it was recognised that cannabis could induce a transient forms of psychosis, which mimicked the symptoms of schizophrenia. Despite this, until the last decade or so, most psychiatrists regarded cannabis as an essentially benign substance.

The media portrayal of Dr Kuepper and her team’s study predominantly lead with generalised declarations, including Those who started smoking the drug at college were 90 per cent more likely to have psychotic symptoms in their mid-20s”. Without clarification this statement may sound extremely damning to non-scientific individuals, suggesting that the majority of individuals who smoke cannabis will develop psychosis. This kind of statement is misleading, even meaningless, if it fails to tell the reader what the risk factors for developing psychosis actually are in the first place. A 90% increase of a very small risk still results in a very small risk. Over the last 60 years, cannabis-use has risen from virtually zero to around 9% of the UK population.However, there has not been an increase in psychotic episodes within the UK population as would be expected if this studies results were correct.

Correlation versus Causality

Another issue with non-scientists understanding of this type of research is the relationship between causality (cause and effect) and correlation (reciprocal relationship). It is very easy to have correlation without causality, so two factors are linked but one does not cause the other.

A classic story that involves correlation without causality occurred in the early part of the twentieth century: the U.S. government put out a warning that there was a correlation between consuming ice cream, and an increase risk in developing polio. To the contemporary reader it would seem obvious that eating ice cream does not result in the contraction of polio. Polio is simply more virulent in the summer, when we also happen to eat more ice cream. Thus, eating ice cream and an increase in polio are correlated, but there is not causality. Correlation does not always link with causality. It is important to bear this in mind in the review of any kind of research. The distinction is important and may make a significant difference.

The Beckley Foundation is not stating that the results of this study are purely correlational; however, in just the same manner, the study cannot claim that their results are causal, as it would seem that the media are portraying them.

Methodological Issues

A large number of potential issues with the methods arise. Indeed, the authors themselves use an entire page to discuss them. Yet none of these issues are found in the press articles.

For example:

“the analyses were not directly adjusted for the possible confounding effects of a family history of psychosis”. Therefore, family history was not sufficiently controlled for in this study.

“In addition, 23% of participants reported lifetime subclinical psychotic symptoms at T2, which is in keeping with the estimated 15-28% rate of subclinical psychotic symptoms in the general population.” 23% of all the participants, not just cannabis smokers, experienced psychotic symptoms during the course of this study, bear in mind that these were 14-24 year olds.

“Finally, as the time between follow-up visits was four years on average, selective recall could have influenced the results. Spurious findings could have arisen if those with psychotic symptoms had better recall of earlier cannabis use.” The participants were asked to recall from the last 4 years any psychotic episodes that they experienced, how good is your memory for events that happened 4 years ago?

Two Molecules are better than one

There is no doubt that the main psychoactive component of cannabis delta-9-tetrahydrocannibinol (THC) causes psychosis in some people. In laboratory studies pure, synthetic THC elicits transient psychosis in 40-50% of healthy people. However, new research shows that the type of cannabis used is an important factor for the development of psychosis.

Street cannabis has changed over the years. So called ‘skunk’ or sinsemilla contains high concentrations of the molecule THC, whilst another constituent called cannabidiol (CBD) has, through selective breeding of plant strains, been eliminated.Remarkably, and in stark contrast to THC, CBD appears to have an anti-psychotic profile. Across animal models, CBD behaves as an anti-psychotic drug. Though few in number at this stage, studies in humans have produced similar findings and given rise to the possibility that CBD might actually be an effective psychiatric medicine, providing a neruro-protective counterpart to the psychosis-inducing THC. It is unique for a plant to contain drug molecules which act on physiological systems in such opposing ways.

Currently there are no methods for controlling the chemical composition of street cannabis. However, a regulated market would enable governments to control the strength and chemical composition of the available cannabis (in a similar way to which alcohol is controlled). This would reduce the harms associated with its use.

A more Rational Approach

Professor Wayne Hall from the University of Queensland and Professor Louisa Degenhardt from the Burnet Institute in Melbourne, questioned the UK’s decision to retain criminal penalties for cannabis use, despite evidence that removing such penalties has little or no detectable effect on rates of use. They advocated an informed cannabis policy, one that “should be based not only on the harms caused by cannabis use, but also on the harms caused by social policies that attempt to discourage its use, such as criminal penalties for possession and use”.

The Beckley Foundation is dedicated to providing original and scientifically validated research into illegal drugs and disseminating the findings, regardless of what they might be, to inform users about the potential harms associated with drug use. Media reporting, intrinsically sensational in its very nature, risks over-sensationalising such studies as this.

The Beckley Foundation advocates rational and evidence-based drug debates that evaluate all facts to gain the ‘complete picture’. Such an approach is vital if we are to move beyond the current stalemate of drug policy.

To view the full report, please click here.

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