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Cannabis could increase risk of psychotic illness later in life by 40 per cent

27 July 2007

Journal Embargo not for publication before: 0001H (UK time) Friday 27 July, 2007

There is now enough evidence to warn young people that using cannabis could increase their risk of developing a psychotic illness later in life, a collaborative Cardiff University study has found.

Cannabis, or marijuana, is the most commonly used illegal substance in most countries, including the UK and USA. In studies over the last decade up to 20 per cent of young people (aged 14-21) in different parts of the world have used cannabis regularly (at least once per week) or used heavily (on more than 100 occasions in total).

Dr Stanley Zammit in the School of Medicine’s Department of Psychological Medicine and colleagues at the Universities of Bristol, Cambridge and Imperial College, London analysed 35 studies dated up to the end of 2006. The researchers assessed the strength of evidence for a relationship between cannabis use and the occurrence of psychotic or other mental health disorders.

The study, funded by the Department of Health, found that individuals who had used cannabis were 41 per cent more likely than those who had never used the drug to have any psychosis (presence of delusions or hallucinations). The risk increased relative to dose, with the most frequent cannabis users more than twice as likely to have a psychotic outcome. Depression, suicidal thoughts, and anxiety outcomes were examined separately, and findings for these outcomes were less consistent, with fewer attempts made to address non-causal explanations than for psychosis.

Dr Zammit, School of Medicine said: "The studies we looked at showed a consistent association between cannabis use and psychotic symptoms, including disabling psychotic disorders such as schizophrenia."

"Despite the inevitable uncertainty, policymakers need to provide the public with advice about this widely used drug. We believe that there is now enough evidence to inform people that using cannabis could increase their risk of developing a psychotic illness later in life."

If having ever used cannabis increases the risk of a psychotic outcome by 41 per cent as indicated by the results of the study, about 14 per cent of psychotic outcomes in young adults in the UK would not occur if cannabis were not consumed.

Notes to Editors:

1.The paper "Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review" by Theresa H M Moore (University of Bristol ), Dr Stanley Zammit (Cardiff University), Anne Lingford-Hughes (University of Bristol), Thomas R E Barnes(Imperial College, London,), Peter B Jones (University of Cambridge), Margaret Burke (University of Bristol), Glyn Lewis (University of Bristol) is published in The Lancet (July 27).

2. Cardiff School of Medicine

The School of Medicine is based at the Heath Park Campus, a site it shares the University Hospital of Wales, the third largest university hospital in the UK. It has an all-Wales role, contributing greatly to promoting, enhancing and protecting the nation’s health in partnership with the National Health Service (NHS) in Wales. The medical curriculum followed at the School enables students to acquire and apply knowledge, skills, judgement and attitudes appropriate to delivering a high standard of professional care. Around 300 new doctors currently graduate from the School every year and the Welsh Assembly Government has invested substantially in new teaching facilities to increase this number further. The School is an international leader in basic and clinically applied research activities and scored highly in the most recent Government Research Assessment Exercise. School of Medicine researchers annually win tens of millions of pounds in research awards to work with Government, the healthcare industries and the charitable sector on the most pressing issues of human health. The School has also been instrumental in establishing and running many important national research initiatives including the Wales Gene Park, Wales Cancer Bank, the Cardiff Institute for Tissue Engineering and Repair and the Healing Foundation UK Centre for Burns Research.

3. Cardiff University
Cardiff University is recognised in independent government assessments as one of Britain’s leading teaching and research universities. Founded by Royal Charter in 1883, the University today combines impressive modern facilities and a dynamic approach to teaching and research. The University’s breadth of expertise in research and research-led teaching encompasses: the humanities; the natural, physical, health, life and social sciences; engineering and technology; preparation for a wide range of professions; and a longstanding commitment to lifelong learning. Cardiff is a member of the Russell Group of Britain’s leading research universities.

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