Continued cannabis use and risk of incidence and persistence of psychotic symptoms: 10 year follow-up cohort study

The abstract of an article published in the March 1st edition of the British Medical Journal:

By Rebecca Kuepper, research psychologist (1), Jim van Os, professor (1), visiting professor (2), Roselind Lieb, professor (3,4), Hans-Ulrich Wittchen, professor (4,5), Michael Höfler, research statistician (5), Cécile Henquet, lecturer (1)

1. Department of Psychiatry and Neuropsychology, South Limburg Mental Health Research and Teaching Network, EURON, Maastricht University Medical Center, Maastricht, Netherlands
2. King’s College London, King’s Health Partners, Department of Psychosis Studies, Institute of Psychiatry, London, UK
3. Department of Psychology, Division of Epidemiology and Health Psychology, University of Basel, Switzerland
4. Max Planck Institute of Psychiatry, Munich, Germany
5. Institute of Clinical Psychology and Psychotherapy, Technical University Dresden, Germany
   

Objective

To determine whether use of cannabis in adolescence increases the risk for psychotic outcomes by affecting the incidence and persistence of subclinical expression of psychosis in the general population (that is, expression of psychosis below the level required for a clinical diagnosis).


Design

Analysis of data from a prospective population based cohort study in Germany (early developmental stages of psychopathology study).


Setting

Population based cohort study in Germany.


Participants

1923 individuals from the general population, aged 14-24 at baseline.


Main outcome measure

Incidence and persistence of subthreshold psychotic symptoms after use of cannabis in adolescence. Cannabis use and psychotic symptoms were assessed at three time points (baseline, T2 (3.5 years), T3 (8.4 years)) over a 10 year follow-up period with the Munich version of the composite international diagnostic interview (M-CIDI).


Results

In individuals who had no reported lifetime psychotic symptoms and no reported lifetime cannabis use at baseline, incident cannabis use over the period from baseline to T2 increased the risk of later incident psychotic symptoms over the period from T2 to T3 (adjusted odds ratio 1.9, 95% confidence interval 1.1 to 3.1; P=0.021). Furthermore, continued use of cannabis increased the risk of persistent psychotic symptoms over the period from T2 to T3 (2.2, 1.2 to 4.2; P=0.016). The incidence rate of psychotic symptoms over the period from baseline to T2 was 31% (152) in exposed individuals versus 20% (284) in non-exposed individuals; over the period from T2 to T3 these rates were 14% (108) and 8% (49), respectively.


Conclusion

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.

Posting of this abstract is for the purposes of research into psychosis.

Also see:

Cannabis use 'raises psychosis risk' - study

Cannabis use 'doubles risk of psychosis for teenagers'

Marijuana Use Linked to Risk of Psychotic Symptoms

Continued cannabis use and risk of incidence and persistence of psychotic symptoms: 10 year follow-up cohort study


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Skunk users face greater risk of psychosis, researchers warn

An article published in today's edition of The Guardian:

By Sarah Boseley, health editor

Skunk, the powerful form of cannabis dominating the street drug market, is seven times more likely to cause psychosis than ordinary cannabis, scientists say.

Dr Marta Di Forti and Prof Robin Murray, who are among the authors of today's paper published in the British Journal of Psychiatry, say skunk is now the same price as ordinary cannabis on the streets of south London, where the study was carried out, and is now the one that is easiest for young people to obtain.

The researchers at the Institute of Psychiatry in London analysed the cannabis consumption of 280 people who were admitted with a first episode of psychosis to the South London and Maudsley NHS Foundation Trust, and compared them with 174 healthy people from the area. Those who used skunk were almost seven times more likely than ordinary cannabis users to develop psychotic illnesses such as schizophrenia, they found.

"In the last five to six years it has been established that there is a link between heavy cannabis consumption and psychosis, but there is a lot of argument about how important it is," said Murray.

Most cannabis users remain healthy, he said, but he pointed to the growing takeover in the street markets of skunk, which contain 18% THC, the substance thought to trigger psychosis, compared with about 4% in cannabis resin (hash).

Forti said 80% of the cannabis sold on the streets of south-east London was now skunk. People who use skunk do not necessarily appreciate its toxicity, she said.

Those in the study had an average age of 25. Those who had psychotic attacks heard voices and had paranoid illusions, such as a conviction that neighbours or family were conspiring against them.

About 1% of the population suffer psychosis at some time in their life. Studies have shown the risk of psychosis doubles in those who use cannabis a few times.

Photograph: Floris Leeuwenberg/Corbis