Amidst the political fireworks in May, it was easy to miss a side skirmish triggered by the government’s decision to reclassify cannabis as a more dangerous Class B drug. According to Colin Blakemore, Professor of Neuroscience at the Universities of Oxford and Warwick anda member of the independent UK Drug Policy Commission, ‘cannabis has become the football in a contest between evidence and passion’.
Dr John Marsden, of the National Addiction Centre at the Institute of Psychiatry, is a chartered psychologist and member of the Advisory Council on the Misuse of Drugs. This body was asked by the Home Secretary to assess the medical and social scientific basis of the classification of cannabis, and the majority of members advised against reclassification. Marsden told us: ‘My personal opinion is that the classification system as it stands currently does not reflect an analysis of the sum of the risks and harms for health for each controlled substance. The decision by ministers to seek to reclassify cannabis reflects political pressures rather than what the available evidence is saying. But I can understand the reasoning and, in truth, the evidence to hand is very far from complete: we need to know a lot more about how people actually smoke strong forms of cannabis and the doses of THC received. But there’s little doubt in my mind that cannabis is a more psychologically riskydrug than many have presupposed.’
In terms of the evidence on consumption, it appears that the Home Secretary’s comments on ‘binge smoking’ are based on a single study published in the International Journal of Drug Policy last year (see tinyurl.com/6o3rqz), which took its samples from cannabis coffee shops in Holland.
Professor Robin Murray, from the Institute of Psychiatry, told The Psychologist that there is general agreement (based on studies of samples seized by the police) that the average strength of the street drug has been ‘creeping up since the late 80s’, and is now maybe four times stronger thanin the 1960s (not 20 times, as some newspapers have reported). But he added that there is still little evidence on smoking habits: ‘It appears that the more frequently you smoke, the more likely you are to smoke skunk. But do people who smoke skunk smoke as much of it?’
Why is there still such a lack of reliable evidence? ‘Because up until recently we thought it was a safe drug’, said Murray. ‘Look at the Lancet editorial in 1995, which gave cannabis a clean bill of health. It’s only since 2002 that a substantial body of longitudinal research has become available. There are now eight such studies showing that people who smoke have a higher risk of later developing psychosis. The best is the oldest - Andréasson et al.’s 1987 study of Swedish army conscripts [see tinyurl.com/4n2pg2]. But we still can’t say it’s a causal risk factor - there’s a lack of understanding over the mechanism. What’s needed now is experimental studies, not epidemiological. I think that will focus on the endocannabinoid system. It may well turn out that some people are just genetically relatively immune to cannabis.’ js
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