In 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 they were 18 had an almost seven-fold increased risk of developing schizophrenia in later life. A New Zealand study, published at the same time, showed that those who started using cannabis by the time they were 15had a four-fold increase in the risk of developing schizophrenia-like illness by the age of 26. The areas of the adolescent brain associated with motivation, impulsivity and addiction are still rapidly developing, which may mean that teenagers are more vulnerable to the addictive and psychotic actions of drugs they take at what is a crucial stage in neural development.
To summarise a wealth of data from all over the world: cannabis use, whatever age you start smoking, is associated with a two-fold increase in later schizophrenia, but adolescent-onset cannabis use is associated with a much higher risk.
It's important to remember that most people who use cannabis do not develop psychosis. However, Professor Robin Murray from the Institute of Psychiatry in London has estimated that the elimination of cannabis use in the UK would reduce the incidence of schizophrenia by about 8 per cent. His figure is based on current cannabis use and may dramatically change if this 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, given steadily increasing rates of use over past decades, why has the incidence of schizophrenia remained relatively stable? Well, if it's cannabis use in early adolescence that is associated with the strongest schizophrenic effect, then we may be sitting on a time bomb that will explode as young users grow up into psychotic adults.
Trends indicate that cannabis use under the age of 16 became prevalent in the early 1990s. One would therefore predict an increase in rates of schizophrenia in the general population over the next 10 years. Indeed, in his review of recent research, published in the British Journal of Psychiatry, Professor Murray argues that there is already evidence that the incidence of schizophrenia is increasing in some areas of London, especially among young people.
If it's the youngest cannabis users who are most at risk then Home Secretary Charles Clarke should be concentrating his efforts primarily on delaying the onset of cannabis use. Given that in Canada each prosecution for cannabis costs $4,500 (pound;1,424), and imprisonment costs an average of $47,000 (Pounds 14,874), surely investing in educational and health promotional approaches targeting the young would be more cost-effective than punitive measures.
Dr Raj Persaud is Gresham professor for public understanding of psychiatry, and director of the Centre for Public Engagement, King's College London.
His latest book is The Motivated Mind (Bantam Press). Email: firstname.lastname@example.org