One of the problems with the way drug policy is debated in this country is that it very many of the comments you hear on the subject either understate or overstate the health risks involved with drugs.
Politicians occasionally get asked whether we have ever taken illegal drugs. In my case I have never had either the desire or the opportunity: unusually for someone of my age, I managed to spend five years at University without ever coming into contact with any drug other than alcohol or nicotine. There was a reason for this. Within weeks of arriving at Bristol University, I entered a debating championship in which contestants could be asked to speak on subjects which might bear no relation to their actual opinions or knowledge, and I was assigned to speak against a motion calling for the legalisation of cannabis.
For the first nineteen years of my life up to that point I had taken little interest in drugs policy. I had heard advocates of legalising cannabis argue that the effects of that drug are no worse than those which can sometimes be caused by worse than those of alcohol and tobacco, and as both booze and fags can kill I was singularly unimpressed by this argument.
Equally, some of the arguments I had heard on the other side sounded a bit alarmist. So I decided to do a little research, and was surprised by how much evidence I quickly managed to find that there were links between smoking pot and mental problems, motor control difficulties, and birth defects in the children of the heavy user.
So I managed to put together a fairly hard-hitting speech against legalisation, which I was subsequently told had earned me a place in the next round. The reaction of several of the audience and other participants after the debate was almost comical: after the debate there were comments to me like "that was incredibly good for a speech defending a viewpoint which nobody could possibly hold; - er - you didn't really believe what you were saying, did you?"
Not knowing how to respond to this - remember, I was nineteen and did not want to make enemies - I recalled the advice attributed to Abraham Lincoln ("Better to remain silent and be thought a fool than to speak out and remove all doubt") and simply smiled. Apparently that was far more intimidating than anything I could possibly have said, and for the rest of my university career most of my fellow students were careful not to say or do anything too incriminating in my presence!
My actual position is to draw a distinction between medicinal and recreational use of cannabis. Responsible use of this substance as a means of pain relief can deliver benefits. I am in favour of making it easier for sufferers from severe pain to legally obtain cannabis for medicinal use under an appropriate degree of professional medical supervision. However, I consider that the scientific evidence that cannabis can also cause harm is sufficient that recreational use should be strongly discouraged. I am therefore against any legal regime that sends the signal that the drug is not dangerous.
Coming back to where we started, an interesting report was published recently in The Lancet concerning the results of studies into the long term effects of cannabis use, and to which several researchers at my old university, Bristol, participated.
The authors, found evidence that using cannabis could increase the risk of developing a psychotic illness such as schizophrenia, later in life.
Dr Stanley Zammit from Bristol and Cardiff Universities, and colleagues at the universities of Bristol, Cambridge and Imperial College London, analysed 35 studies dated up to 2006. They assessed the strength of evidence for a causal relationship between cannabis use and the occurrence of psychotic or other mental health disorders.
The study, funded by the Department of Health and based in the University of Bristol, found that individuals who used cannabis were 41 per cent more likely to have any psychosis than those who had never used the drug. The risk increased relative to dose, with the most frequent cannabis users more than twice as likely to have a psychotic outcome.
Professor Glyn Lewis from the University of Bristol, and senior author on the paper, 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."
Dr Zammit added: "Policymakers want to provide the public with advice about this widely used drug. However, even if cannabis does cause an increase in risk of developing psychosis most people who use cannabis will not develop such an illness. 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."
The authors estimate that if cannabis had a causal relationship with psychosis, about 14 per cent of psychotic illnesses in young adults in the UK could be prevented if cannabis were not consumed.