Alarming new research shows smoking cannabis can trigger psychotic illness such as schizophrenia almost three years earlier.
A new report which analysed 83 studies involving 22,000 people found people vulnerable to mental health problems are likely to suffer symptoms at a much younger age by using the illicit drug.
The findings confirm that cannabis accelerates the onset of full-blown mental illness, which makes it harder to treat successfully.
Researchers in Australia behind the latest analysis in the Archives of General Psychiatry journal and British experts are calling for renewed public health warnings about the dangers of cannabis.
They claim ‘overwhelming evidence’ shows it triggers schizophrenia and other psychotic disorders, perhaps through an interaction between genetic and environmental disorders or by disrupting brain development.
Dr Matthew Large and colleagues at the Prince of Wales Hospital, New South Wales, looked at 83 studies involving 8,000 people who used cannabis and other substances and 14,000 people who did not.
They found those who used cannabis developed psychosis about 2.7 years younger than those who did not.
Those who used any type of substance developed psychosis about two years younger, whereas the use of alcohol did not affect the age at which psychosis began.
Typical psychotic events among cannabis users are paranoid ideas, hallucinations, hearing voices or bizarre behaviour.
Dr Large said ‘The results of this study provide strong evidence that reducing cannabis use could delay or even prevent some cases of psychosis.
‘Reducing the use of cannabis could be one of the few ways of altering the outcome of the illness because earlier onset of schizophrenia is associated with a worse prognosis and because other factors associated with age at onset, such as family history and sex, cannot be changed.
‘The results of this study confirm the need for a renewed public health warning about the potential for cannabis use to bring on psychotic illness.’
Previous research shows one-quarter of the population has a strong genetic predisposition to develop psychosis and other schizophrenia-like symptoms after smoking cannabis.
The risk increases more than 10-fold in these people.
Experts warn that anyone with pre-existing mental health problems or family history is at increased risk of mental illness if they use cannabis.
Mary Brett, a trustee of CanSS (Cannabis Skunk Support), said the study’s conclusions provided ‘overwhelming evidence’ of the link between cannabis use and psychosis.
She said ‘We know from research done by the Institute of Psychiatry that adolescents who carry a particular copy of a gene are five times more likely to develop psychosis after smoking cannabis - 10 times if they carry two copies.
‘We’ve been trying to campaign for more awareness and a public health campaign is long overdue.
‘People are not listening to the evidence that psychosis happens quicker and younger to cannabis users, and it’s heartbreaking to see the consequences for individuals and families.’
It was reported yesterday that respected GP Dr Hans-Christian Raabe claims he was sacked as a government drugs adviser for holding ‘embarrassing’ views about homosexuality.
Dr Raabe, a ‘traditional Christian’, was appointed to the Advisory Council on the Misuse of Drugs less than a month ago and has not had the chance to attend a meeting.
Home Office officials said he had failed to disclose being a co-author of a study suggesting a link between homosexuality and paedophilia.
The Advisory Council was the centre of controversy in 2009 when its chairman David Nutt was sacked for insisting cannabis was less harmful than alcohol and tobacco.