Cannabis Culture

PROFESSOR RAJ PERSAUD explains the dangers of cannabis, and considers why users are uninhibited by the health risk

An enigma frequently clouds the cannabis debate: if it's as dangerous as doctors and scientists claim and possibly the most used illicit drug worldwide, why do the ill effects appear to affect so few?

Surveys suggest that as many as one in four of those aged between the late and early twenties in the UK admit to having smoked cannabis recently. Yet the rate of schizophrenia remains relatively low in comparison - roughly one in a hundred.

Previously the debate over the dangers of cannabis had focused on other controversial areas such as how dependency inducing it was and the physical damage But now psychiatrists in particular are concerned at the accumulating evidence that cannabis produces devastating effects on mental health in the form of psychosis.

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.

So one possible answer to the enigma of widespread use combined with apparently low incidence of mental health effects could be that the age at which you start smoking that is a crucial mediating factor. We know the adolescent brain is developing rapidly and could be particularly vulnerable to damage if psychoactive substances are imbibed during a "critical period" or "window" of brain development.

Another possible answer is that smoking cannabis if you are genetically predisposed to psychosis produces a very different mental health outcome compared to if you have a contrasting genetic template. The genes load the gun but it's the cannabis which pulls the trigger. Given you don't know your own genetic endowment (the blood test is only available at some specialist research centres including the Institute of Psychiatry in London), smoking cannabis is a lot closer to playing Russian roulette than many realise.

Professor Robin Murray, from the Institute of Psychiatry has recently 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%.

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.

Cannabis use under the age of 16 years is a fairly new phenomenon, appearing 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, Professor Murray argues that there is already some evidence that the incidence of schizophrenia is increasing in certain areas of London, especially among young people.

Yet in a sense all the statistics or data in the world make little difference to the cannabis debate for one key psychological reason - we have a natural human tendency to be poor at assessing risk when it is presented to us in the form of numbers. Our brains are wired up much more to making decisions over risk in real world situations - we make assessments from our direct experience.

Few will directly experience psychosis either in themselves or others. Even those we are in daily contact with, friends and relatives, may not advertise severe mental illness if they develop it. The purpose of mental health services is to offer privacy and efficient treatment away from prying eyes - the resulting taboo ensures we are only dimly aware of the true mental health of our neighbours or colleagues.

It therefore becomes difficult to engage with the actual hazards of cannabis if we are using our direct experience, particularly if that is of several acquaintances using the drug apparently without ill effect. Another intriguing social psychological effect comes into play here: our tendency to select friends and acquaintances so that they may assist us with choices we are already predisposed to take.

It was previously thought that peer pressure played a considerable role in determining which adolescents ended up taking drugs and which desisted. Falling in with a "bad crowd" could be fatal. Yet the latest research now suggests that the drug abuse pathway is more complicated. It appears we select our friends in a way that reflects our personality. So, for example, risk-takers tend to choose other risk-takers as friends, and it is the interaction between them and our own predisposed personality that results in the drug taking behaviour, amongst other outcomes. So it is not that we fall in with the wrong crowd - more that we choose the wrong crowd because, at some level, their choices appeal to us.

This offers an important lesson: when we assess risk - as in say the cannabis debate - perhaps we should assess our social environment and direct experience which will most influence us, rather than merely studying the statistics. Have we chosen to be surrounded by those whose own choices will merely reinforce our own?

Sometimes the best way to improve our decision-making is to actively seek out and experience those aspects of the debate we may tend to avoid because it may be inconvenient to our accepted outlook. For this reason I wish it was possible for more to experience my own ward at the Bethlem Royal and Maudsley NHS Hospitals Trust where cannabis abuse appears an epidemic. It has lit a fuse, the explosion from which, in the form of potentially dramatically higher rates of schizophrenia, could have massive fall out, affecting us all.

Dr Raj Persaud is Gresham Professor for Public Understanding of Psychiatry and Consultant Psychiatrist at The Maudsley Hospital in South London