Cannabis 'can double chances of psychotic illness': Reports

Smoking cannabis on a regular basis can more than double the chances of suffering psychotic illnesses such as schizophrenia, new research in the UK has shown.

Any use of cannabis, which means even taking the drug just once, was associated with a 41% greater risk.

The findings emerge from the most comprehensive analysis yet carried out of the evidence linking cannabis use and psychosis.

Scientists in the UK pooled data from 35 different studies which together showed a clear dose-response effect.

People who smoked the most cannabis were the most likely to suffer a psychotic breakdown marked by delusions, hallucinations or disordered thoughts.

For frequent users, the risk rose to between 50% and 200%.

The research, published today in The Lancet medical journal, implies that 14% of psychotic illness now affecting young adults in the UK could be prevented if no one took cannabis.

Two Danish experts commenting on the research in the journal wrote: “This finding has tremendous implications for young people, their families, and society.”

About 20% of young adults claim to take cannabis at least once a week and around 40% are believed to have tried it at some time in their lives.

The study is likely to have a big impact on the Government’s deliberations about cannabis.

British Prime Minister Gordon Brown is now considering whether cannabis should be returned to its previous status as a class B drug. Under Tony Blair’s premiership, the drug was downgraded from class B to class C, so possession ceased to be an arrestable offence.

Alternatively, a class B rating could be reserved just for the extra-potent form of cannabis known as “skunk”.

The new research was funded by the Department of Health and based at the University of Bristol.

Its authors said there was no definite evidence cannabis triggered psychosis, only that an association existed.

However, the link was so strong it warranted warnings to the public.

Senior author Professor Glyn Lewis, from the University of Bristol, said: “It is difficult to be certain about whether cannabis use causes psychotic illnesses such as schizophrenia. It is possible that the people who use cannabis might have other characteristics that themselves increase risk of psychotic illness. However, all the studies have found an association and it seems appropriate to warn members of the public about the possible risk.”

Colleague Dr Stanley Zammit, from Bristol and Cardiff universities, pointed out that even if cannabis did increase the risk of psychosis, most people using the drug would not get ill.

But he added: “Nevertheless, we would still advise people to avoid or limit their use of this drug, especially if they start to develop any mental health symptoms or if they have relatives with psychotic illnesses.”

At present there was little to indicate that cannabis use increased the risk of depression, suicidal thoughts or anxiety, said the researchers.

Danish experts Merete Nordentoft and Carsten Hijorthoj calculated that taking cannabis might account for 800 new cases of schizophrenia among 15 to 34-year-olds in the UK each year, assuming a causal relationship.

The two psychiatrists, from Copenhagen University Hospital, wrote in their commentary: “In the public debate, cannabis has been considered a more or less harmless drug compared with alcohol, central stimulants, and opioids. However, the potential long-term hazardous effects of cannabis with regard to psychosis seem to have been overlooked, and there is a need to warn the public of these dangers, as well as to establish treatment to help young frequent cannabis users.”

Marjorie Wallace, chief executive of the mental health charity Sane, said: “The Lancet report justifies Sane’s campaign that downgrading a substance with such known dangers masked the mounting evidence of direct links between the use of cannabis and later psychotic illness.

“This analysis of 35 studies should act as a serious warning of the dangers of regular or heavy cannabis use, doubling the risk of developing later schizophrenia, a condition in which a person may hear voices, and experience strange thoughts and paranoid delusions.

“The debate about classification should not founder on statistics but take into account the potential damage to hundreds of people who without cannabis would not develop mental illness.”

A spokesman for the Transform Drug Policy Foundation, which campaigns for drug law reform, warned against “knee-jerk reactions” that could lead to bad policy.

He said the absolute risk of psychosis was tiny in relation to the estimated 6.2 million people in the UK who took cannabis.

Paul Corry, director of public affairs at mental health charity Rethink, said: “This research backs up our call for an ongoing education campaign so that young people and their parents are aware of the risks. How much more research is needed before our call is answered? Rather than focusing its attention on the reclassification debate, the Government would do well to crack on with the more important job of informing the public about the health implications.”

Shadow British Home Secretary, David Davis, said: “This is yet more evidence that starkly demonstrates the harm the Government’s chaotic and confused policy is causing.

“Not only is cannabis a gateway to harder drugs and a major cause of crime, it has real and tragic consequences for the mental health of so many people.

“On the basis of the scientific and medical evidence available alone, the Government should reverse their disastrous policy of declassifying this harmful drug.”