That Charles Clarke should be having second thoughts about cannabis appears puzzling, as medical research has long confirmed that it is not as safe a drug as occasional middle-class users are inclined to believe.
Even before we get into the debate over the link between cannabis and psychosis, let's not forget that marijuana smoke contains similar levels of tar as tobacco smoke and up to 50 per cent more carcinogens. Cannabis users also smoke unfiltered material, inhale more deeply, and hold the smoke longer than tobacco smokers, resulting in substantially greater tar deposits in the lungs than tobacco users.
In one recent study, heart attacks were found to be more than four times as likely to occur in the hour after smoking cannabis. If cigarettes can be calculated to cause an annual excess of 120,000 deaths for every 13 million smokers, then the corresponding excess mortality for that number of cannabis users would be approximately the same. In a UK study of fatal road accidents, no alcohol was found in 80 per cent of people found positive for cannabis at autopsy.
Given that cannabis is the most widely used illicit drug in the world, the lack of official interest in educating the public about its dangers remains mysterious. As this neglect may be to blame for the rise in cannabis use, particularly among the young, it is the implications of this increase that may be actually what is troubling Mr Clarke, not the "pretext" of new medical research.
As far back as 2002 a large-scale study of more than 50,000 men conscripted into the Swedish army between 1969 and 1970 suggested that those who had used cannabis more than 50 times before the age of 18 years had an almost sevenfold increased risk of developing schizophrenia in later life. In a New Zealand study published at the same time, those who started cannabis use by age 15 years (but not those who started later) showed a fourfold increase in the risk of developing schizophrenia-like illness by age 26 years.
What psychiatrists were now becoming aware of for the first time was that the age when cannabis is first smoked was emerging as a crucial risk factor in later development of psychosis.
We now know that the adolescent brain regions associated with motivation, impulsivity, and addiction are still rapidly developing, probably conferring greater vulnerability to the addictive and psychotic actions of drugs taken at this crucial neural development stage.
To summarise a wealth of data from all over the world: cannabis use, whatever your age of smoking, is associated with a twofold increase in later schizophrenia, but adolescent-onset cannabis use is associated with a much higher risk.
Yet the vast majority of young or older people who use cannabis do not develop psychosis.
Professor Robin Murray, from the Institute of Psychiatry, has attempted to simplify the statistics. In the most recent comprehensive review of the research he estimates that the elimination of cannabis use in the UK would reduce the incidence of schizophrenia by approximately 8 per cent.
That figure might dramatically change if cannabis use goes up, even more strikingly if it increases in the young. Ominously the number of cannabis users seeking treatment has doubled in the past 10 years in the UK.
Yet if cannabis causes psychosis, and if its use has steadily risen over past decades, how come the incidence of schizophrenia in the population has remained relatively stable? Marijuana use in early adolescence is associated with the strongest schizophrenic effect, so perhaps what is really worrying Mr Clarke, quite rightly, is the possibility that we are sitting on a schizophrenia time-bomb, which will blow up in our faces as young cannabis users grow up into psychotic adults.
Trends of cannabis use among adolescents indicate that use under the age of 16 years is a fairly new phenomenon that has appeared only since the early 1990s. One would therefore predict an increase in rates of schizophrenia in the general population over the next 10 years. Indeed, there is already some evidence that the incidence of schizophrenia is currently increasing in some areas of London, especially among young people, argues Professor Murray in his recent review.
But if it is the youngest cannabis users who are most at risk, perhaps because their cannabis use is most likely to become long-standing, then Mr Clarke should surely be concentrating his efforts primarily on delaying the age at which cannabis is first used.
In Canada it is calculated that each prosecution for cannabis costs $4,500, and imprisonment costs an average of $47,000. So rather than wringing hands about how punitive to be about cannabis, surely investing this kind of money in educational and health promotional campaigns targeting the young would be more cost-effective?
Dr Raj Persaud is director of the Centre for Public Engagement with Mental Health Sciences at the Institute of Psychiatry, King's College LondonReuse content