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expert reaction to daily cannabis use, potency and psychosis

Research published in Lancet Psychiatry demonstrates that daily use of high potency cannabis is linked to higher rates of psychosis.

Dr Amir Englund, Post-doctoral researcher in Psychopharmacology, Institute of Psychiatry Psychology & Neuroscience, King’s College London (IoPPN), said:

“This is a very impressive and important study as it confirms the findings of previous studies which show high-potency cannabis carries a greater risk for psychosis compared to weaker types. It also shows that the variation of the strength of cannabis explains some of the differences in rates of psychosis across Europe (and Brazil), whereby places with stronger forms of cannabis have higher rates of psychosis. Therefore, it might be possible to reduce the rates of psychosis by reducing frequent use of high-potency cannabis.

“The study is a case-control study which means that the frequency of use and type of cannabis used (high or low potency) was compared between those who have a psychotic disorder and healthy people. Patients were far more likely to be using more often and higher potency cannabis compared to healthy controls, even after controlling for social factors and other drug use – which could explain some of the associations. However, studies such as these are unable to say whether it is the frequent use of stronger cannabis that is the cause of these patients having psychosis, but it also cannot be ruled out. It may be that cannabis causes psychosis for some, or that patients with psychosis prefer stronger cannabis, or both.

“Since the issue of causality is likely never to be definitively proven – we must assume the worst and try to reduce the frequent use of stronger forms of cannabis, especially among those who have a vulnerability towards psychosis.

“Although this study should be considered in policy debates about cannabis, it does not help us answer the question if legalising or prohibiting cannabis is the best course of action. It is important to remember that cannabis potency increased across the world mainly during prohibition. However, in US states where cannabis is now legal but with very few restrictions, the potency of cannabis has further increased and is even higher than the high-potency cannabis of the black market. We need a less politicised debate about how we can effectively reduce the availability and use of more harmful forms of cannabis.”

Dr Michael Bloomfield, Excellence Fellow, Head of Translational Psychiatry Research Group and Consultant Psychiatrist, University College London (UCL), said:

“This is important and well conducted research which adds to a growing body of evidence which indicates that heavy cannabis use with high levels of THC increases the risk of psychosis. This research adds weight to the advice that people who use cannabis recreationally should avoid high THC cannabis and that care is needed when treating patients with cannabis-derived products.”

Prof David Nutt, The Edmond J Safra Chair and Head of the Centre for Neuropsychopharmacology, Division of Brain Sciences, Dept of Medicine, Imperial College London, said:

“These are interesting findings that support the campaign for making low d9THC cannabis (the psychoactive ingredient) strains readily available for recreational use, perhaps because they also contain the antipsychotic element cannabidiol.

“It is also important to realise that d9THC is well known to produce psychosis in healthy volunteers i.e. people without any predisposition to mental illness. Indeed, it has been used for this purpose for many years and is one of the best models of psychosis we have.  This efficacy is more marked for skunk than for balanced mixtures.  This means that many of these cases of psychosis may be acute cannabis induced psychosis rather than some other form of psychosis such as schizophrenia. The final outcome of the psychosis diagnosis cannot be confirmed at present – in general drug induced psychosis have a more benign course than schizophrenia – we must not conclude that cannabis causes schizophrenia from these data.

“Other very important points to note are the significantly higher level of other psychotomimetic drug use in this group [stimulants, ketamine, hallucinogens], substances which also are used to produce psychosis in healthy volunteers.  The significantly different ethnicity and cultural backgrounds and educational and employment backgrounds of those diagnosed with psychosis in this study are significant confounders too. All these factors are likely relevant too and may indicate both predisposing factors to cannabis [and other drug] use and the challenge of making diagnoses in people from different cultures and languages.”

Dr Adrian James, Registrar at the Royal College of Psychiatrists said:

“This is a good quality study and the results need to be taken seriously.

“Cannabis carries severe health risks and users have a higher chance of developing psychosis. The risks are increased when the drug is high in potency, used by children and young people and when taken frequently. Because of these risks, a good drugs strategy should focus on preventing and reducing harm, not on diverting people to the criminal justice system.

“One way that the harm caused by substance addiction can be reduced is restoring accessible addiction services that are well provisioned and staffed – this needs to be addressed urgently.

“The Royal College of Psychiatrists is reviewing the mental health effects of cannabis use. We hope that a rounded picture of the biological, psychological and social effects of cannabis will give policy makers the knowledge needed to put the health and wellbeing of our patients at the centre of their decisions.”

‘The contribution of cannabis use to variation in the incidence of psychotic disorder across Europe (EU-GEI): a multicentre case-control study’ by Marta Di Forti et al. was published in Lancet Psychiatry at 23:30 UK time on Tuesday 19th March.

All our previous output on this subject can be seen at this weblink:

Declared interests

Dr Michael Bloomfield: No conflicts of interest

Prof David Nutt: No conflicts of interest

None others received.

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