New Times,
New Thinking.

  1. Long reads
3 September 2007

An epidemic of cannabis use?

Dr Raj Persaud warns against the dangers of an epidemic of cannabis use - which he argues could have

By Raj Persaud

An enigma which frequently clouds the cannabis debate is – if it’s as dangerous as doctors and scientists claim – how come despite being possibly the most used illicit drug worldwide, the ill effects appear to affect so few?

For example surveys suggest that as many as one in four of those aged from the late teens to the early twenties in the UK admit to having smoked cannabis recently – yet the rate of schizophrenia remains relatively but stubbornly 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 what was the physical damage, but now psychiatrists in particular are concerned at the accumulating evidence 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 is that it could be 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.

Select and enter your email address Your weekly guide to the best writing on ideas, politics, books and culture every Saturday. The best way to sign up for The Saturday Read is via The New Statesman's quick and essential guide to the news and politics of the day. The best way to sign up for Morning Call is via
  • Administration / Office
  • Arts and Culture
  • Board Member
  • Business / Corporate Services
  • Client / Customer Services
  • Communications
  • Construction, Works, Engineering
  • Education, Curriculum and Teaching
  • Environment, Conservation and NRM
  • Facility / Grounds Management and Maintenance
  • Finance Management
  • Health - Medical and Nursing Management
  • HR, Training and Organisational Development
  • Information and Communications Technology
  • Information Services, Statistics, Records, Archives
  • Infrastructure Management - Transport, Utilities
  • Legal Officers and Practitioners
  • Librarians and Library Management
  • Management
  • Marketing
  • OH&S, Risk Management
  • Operations Management
  • Planning, Policy, Strategy
  • Printing, Design, Publishing, Web
  • Projects, Programs and Advisors
  • Property, Assets and Fleet Management
  • Public Relations and Media
  • Purchasing and Procurement
  • Quality Management
  • Science and Technical Research and Development
  • Security and Law Enforcement
  • Service Delivery
  • Sport and Recreation
  • Travel, Accommodation, Tourism
  • Wellbeing, Community / Social Services
Visit our privacy Policy for more information about our services, how Progressive Media Investments may use, process and share your personal data, including information on your rights in respect of your personal data and how you can unsubscribe from future marketing communications.

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 lot closer to playing Russian Roulette than many realise.

To summarise a wealth of data from all over the world: cannabis use, whatever your age of smoking, is associated with a general twofold increase in later schizophrenia, but adolescent-onset cannabis use is associated with a much higher risk.

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 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.

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 in a sense all the statistics or data in the world may 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 its presented to us in the form of numbers or data. Our brains are wired up much more to making decisions over risk in actual 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, if they develop severe mental illness they may not advertise it. Part of the direct purpose of mental health services is to offer privacy and efficient treatment away from prying eyes – so the taboo surrounding the area conspires to ensure we are only dimly aware of the true mental health of our neighbours or colleagues.

The actual hazards of cannabis are therefore become difficult to engage with 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 and that is our tendency to select our friends and acquaintances so that they in turn may assist us with choices we are already predisposed to take.

For example it was previously thought that peer pressure played a considerable role in determining which adolescents ended up taking drugs and which desisted successfully. Falling in with a ‘bad crowd’ could be fatal in this respect. Yet the latest research now suggests that actually the mechanism by which a drug abuse pathway in life is followed is more complicated than that. It turns out we appear to 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’s this interaction between them and our own predisposed personality, which results in the drug taking behaviour, amongst other outcomes.

So its 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 has an important lesson for us – when we assess risk – as in say the cannabis debate – perhaps we should assess our social environment as well as its this direct experience – not the statistics – which will most influence us. 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 own accepted outlook. For this reason I just 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 and has lit a fuse the explosion from which, in the form of possibly dramatic higher rates of schizophrenia in the near future, 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

Content from our partners
Peatlands are nature's unsung climate warriors
How the apprenticeship levy helps small businesses to transform their workforce
How to reform the apprenticeship levy