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UK: Panel Says Link With Mental Illness Is 'Very Small'
James Randerson The Guardian
Saturday 14 Jan 2006 New scientific evidence suggests a causal link between cannabis use and long-term psychotic symptoms, according to the government's top drug advisory committee. But in a draft report to the home secretary, Charles Clarke, seen by the Guardian, the committee says that the risks are not high enough to support reclassification as class B. The report says: "The [committee] considers that cannabis products should remain class C. At worst, the risk to an individual of developing a schizophreniform illness as a result of using cannabis is very small. The harmfulness of cannabis, to the individual, remains substantially less than the harmfulness caused by substances currently controlled under the act as class B." A source close to the committee said only one member out of 36 voted to shift cannabis back to class B. The report said: "Collectively, the weight of evidence from these studies suggests an association between cannabis use, and the development of schizophrenia or a schizophreniform disorder, that is causal ... The best current estimate of the populations-attributable risk of schizophreniform illness due to cannabis (the percentage reduction in the prevalence of the condition if the use of cannabis could be totally eliminated) is about 8-10%." In March, Mr Clarke asked the Advisory Council on Misuse of Drugs (ACMD) to look at the decision to declassify cannabis from B to C and consider evidence that cannabis available in Britain is increasing in potency. He directed it to two recent population-based studies suggesting a link between cannabis use and mental health problems. On the potency question, the ACMD concluded: "There is no evidence that, during this period [1995-2005], the potency of cannabis resin has changed in any significant way ... There has, however, been an increase in the potency in sinsemilla." This refers to intensively grown super-strength varieties such as skunk. Since 2002 a handful of population-based studies have pointed to cannabis being a causal factor in schizophrenia and the development of more subtle psychotic symptoms. "Each one of these studies has its own deficiencies, but the impressive thing is that they all point in the same direction," said Robin Murray, a psychiatrist at King's College London who wrote some of the papers. They suggest that cannabis increases the chance of a user developing psychotic symptoms two-or fourfold, he said. The risk of lung cancer for cigarette smokers is 10-15 times. He believes that reclassifying cannabis was a mistake, but he accepts that the scientific case is far from watertight. Cannabis use and schizophrenia could be linked by a third factor, perhaps cannabis users being more likely to use another drug that causes mental health problems, something the new studies try to take into account. Others are sceptical of the new studies. "The total number of people involved is very, very small," said Professor Leslie Iversen, a pharmacologist at Oxford University and ACMD member. "If you look at any of those studies, a very tiny proportion of the cohort actually develop psychotic illness that might be attributable to cannabis." Research by Cecile Henquet and her colleagues at the University of Maastricht was also considered by the council. "The consensus is that cannabis is not harmless. It is much more harmful than we expected, but it is not that cannabis in itself can cause psychosis," she said. "Apparently some people have a sensitive brain to cannabis exposure." GROUP TRIALS Study group: Swedish army conscripts Size: Around 50,000 When: Originally 1988, but updated in 2002 Finding: Heavy cannabis users at 18 years old were 6.7 times more likely than non-users to be diagnosed with schizophrenia 27 years later. ----- Study group: Netherlands Mental Health Survey Size: Around 4,000 When: 2002 Finding: Cannabis users nearly three times more likely to experience psychotic symptoms 3 years later. ----- Study group: New Zealand cohort studied for 20 years Size: Around 1,000 When: 2003 Finding: Users dependent on cannabis at 18 were 3.7 times more likely to get psychotic symptoms. ----- Study group: Individuals born in Dunedin, New Zealand Size: Around 750 When: 2002 Finding: 10.3% of cannabis users aged 15 were diagnosed with schizophrenia-like conditions at 26, against 3% of non-users.
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