Cannabis Could Increase Risks Of Psychotic Illness By 40 Percent

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 causal 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.