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Psychosis 'linked to cannabis use'


There is a warning that the risk of psychosis is increased if cannabis is used as a teenager or young adult.

The link between cannabis and psychosis has been heavily debated, with questions over whether the connection is casual, or if early psychotic experiences lead people to “self-medicate” with the drug.

The latest research comes from data of more than 1,900 people who took part in a study, aged between 14 to 24 at the start.

Researchers re-examined those involved three and eight years later to analyse the connection between cannabis use and psychotic symptoms.

They found that the volunteers who did not use the drug at the start of the study, but went on to start using, had a higher risk of psychotic symptoms later on.

In those who were using cannabis at the start of the study and carried on, there was also an increased risk of psychotic experiences.

The experts, including from Germany, the Netherlands and the Institute of Psychiatry in London, published their findings in the British Medical Journal (BMJ).

They concluded: “Cannabis use is a risk factor for the development of incident psychotic symptoms.

“Continued cannabis use might increase the risk for psychotic disorder by impacting on the persistence of symptoms.”

The experts found no evidence that psychotic symptoms led to later cannabis use.

Sir Robin Murray, professor of psychiatric research at the Institute of Psychiatry, said: “This study adds incremental information to the already fairly solid evidence that continued use of cannabis increases risk of psychotic symptoms and psychotic illness.”

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Readers' comments (3)

  • It's worth looking closely at this study because very quickly the incredibly weak nature of its conclusions become clear.

    It’s not about clinically diagnosed psychosis at all. It’s about what may be just one trivial thought or mental confusion in the space of 10 years. The authors call it "subclinical expression of psychosis in the general population...that is, expression of psychosis below the level required for a clinical diagnosis." Astonishingly, that's enough for a "positive".

    Once you delve behind the headlines, phrases like “might under certain circumstances” start to appear and then you realise how meaningless the study’s conclusions are.

    Something else that nearly all the reports of this study have missed out is the authors statement that "The evidence on cannabis and psychosis has influenced the decision in the UK to retain criminal penalties for cannabis use, despite evidence that removing such penalties has little or no detectable effect on rates of use. An informed cannabis policy 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."

    Putting Cannabis "Research" Into Perspective. See here:

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  • As an experienced RMN it is of no doubt to me that the effects of cannabis use on our client group is detrimental. Through the years I have the seen the damage all too frequently in the acute admissions setting. What frustrates me most is why people insist on having this debate at all. If people could only see, first-hand, the actual risks and harm cannabis all too often causes, in the real world, they might not be so opposed to discouraging its use.

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  • I too am an RNMH and frequently observe the contribution of cannabis as one of a constellation of factors involved in the development of pychosis, however I believe we need to be very careful not to go down the daily mail route of overemphasising the links. It is true that for a small number of vulnerable individuals there is a risk but for a large majority of individuals there is no association with psychosis.

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