Cannabis 'doubles risk of psychotic illness'

The strongest evidence of the link between cannabis and serious mental illness was disclosed last week in a study showing the drug is responsible for one in seven psychotic disorder cases in young adults.

Those who smoke the drug regularly are more than twice as likely to suffer illnesses such as schizophrenia, hallucinations and delusions later in life, scientists found.

The findings from the British Government-funded study, published in The Lancet, come days after a review of the law on cannabis was ordered, which could see the drug reclassified as Class B. It was downgraded three years ago to Class C, which carries lesser penalties for possession and dealing. Cannabis is the most popular drug in Britain, with 11 per cent of those aged 16 to 59 saying they had used it in the past year. Up to one in five young people now reports smoking it at least once a week.

Case study

In the study, which reviewed 35 studies going back to the 1960s, researchers found that young people who smoked cannabis were up to 41 per cent more likely to develop a psychotic disorder than those who had never tried it. The more they smoked, the higher the risk, with the most frequent users more than twice as likely to have psychotic experience.

Dr Theresa Moore, from Bristol University, and Dr Stanley Zammit, from Cardiff University, who carried out the research, said it was vital that young people were made aware of the dangers. "We have described a consistent association between cannabis use and psychotic symptoms, including disabling psychotic disorders," they said.

"We believe there is now enough evidence to inform people that using cannabis could increase their risk of developing a psychotic illness later in life."

In an accompanying editorial, Dr Merete Nordentoft and Dr Carsten Hjorthaj from Copenhagen University Hospital, said that about 800 cases of schizophrenia in Britain could be prevented every year if people stopped smoking cannabis.