Blah blah cannabis blah blah blah

July 28th, 2007 by Ben Goldacre in drurrrgs | 67 Comments »

The Guardian
Saturday July 28 2007

You know when cannabis hits the news you’re in for a bit of fun, and this week’s story about cannabis causing psychosis was no exception. The paper was a systematic review and then a “meta-analysis” of the data which has already been collected, looking at whether people who smoke cannabis are subsequently more likely to have symptoms of “psychosis” or diagnoses of schizophrenia. Meta-analysis is, simply, where you gather together all of the numbers from all the studies you can find into one big spreadsheet, and do one big calculation on all of them at once, to get the most statistically powerful result possible.

Now I don’t like to carp, but it’s interesting that the Daily Mail got even these basics wrong, under their headline “Smoking just one cannabis joint raises danger of mental illness by 40%”. Firstly “the researchers, from four British universities, analysed the results of 35 studies into cannabis use from around the world. This suggested that trying cannabis only once was enough to raise the risk of schizophrenia by 41%.”

In fact they identified 175 studies which might have been relevant, but on reading them, it turned out that there were just 11 relevant papers, describing seven actual datasets. The Mail made this figure up to “35 studies” by including 24 separate papers which the authors also found on cannabis and depression, although the Mail didn’t mention depression at all.

They also said that “previous studies have shown a clear link between cannabis use in the teenage years and mental illness in later life”. They then described some of these previous studies. These were the very studies that are summarised in the new Lancet paper.

But what was left out is as interesting as what was added in. The authors were clear – as they always are – that there were problems with a black-and-white interpretation of their data, and that cause and effect could not be stated simply. For ongoing daily users, as an example, it’s difficult to be clear that cannabis is causing people to have a mental illness, because their symptoms may simply be due to being high on cannabis all the time. Perhaps they’d be fine if they were clean.

It was also interesting to see how the risk was numerically reported. The most dramatic figure is always the “relative risk increase”, or rather: “cannabis doubles the risk of psychosis”, “cannabis increases the risk by 40%”. Because schizophrenia is comparatively rare, translated this into real numbers this works out – if the figures in the paper are correct, and causality is accepted – that about 800 yearly cases of schizophrenia are attributable to cannabis. This is not belittling the risk, merely expressing it clearly.

But what’s really important, of course, is what you do with this data. Firstly, you can mispresent it, and scare people. Obviously it feels great to be so self-righteous, but people will stop taking you seriously. After all, you’re talking to a population of young people who have worked out that you routinely exaggerate the dangers of drugs, not least of all with the ridiculous “modern cannabis is 25 times stronger” fabrication so beloved by the media and politicians.

And craziest of all is the fantasy that reclassifying cannabis will stop six million people smoking it, and so eradicate those 800 extra cases of psychosis. If anything, for all drugs, increased prohibition may create market conditions where more concentrated and dangerous forms are more commercially viable. We’re talking about communities, and markets, with people in them, after all: not molecules and neuroreceptors.

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67 Responses

  1. richard_p_auckland said,

    July 30, 2007 at 10:57 pm

    I was trying to be humorous – for those that don’t know, Dunedin is a university town in the South Island of New Zealand. It’s noted for the hard partying lifestyle of the students.

  2. manigen said,

    July 30, 2007 at 11:30 pm

    Of course, strictly speaking, I should have said Ben’s popular.

    Or in fact, Ben’s swearing is popular.

    Or, you know, Dr Stott’s swearing as reported.

  3. Dr Aust said,

    July 31, 2007 at 11:10 am

    Sigh. Another of those issues where whatever the scientific / medical truth, it will always be submerged by a ton of scare stories and political posturing.

    As rs was sort of alluding to above, it seems to me that without detailed and accurate information on HOW MUCH cannabis the people in the studies get through the whole discussion is largely meaningless…

    Also add the difficulty of controlling for “self-medication”, the problems of ruling out the (quite common) psychotic episodes that aren’t ultimately diagnosed as schizophrenia, and the “population factors” (dope smoking highest among adolescent males, earliest pyschotic episodes typical in the early 20s in men even if not smokers, etc etc)

    …and you have a right mess.

    I’m all for advising teenagers not to fry their brains daily with 2 hrs relentless toking on the hydroponic skunk, but the moral panic about the stuff is all too “reefer madness” for words.

    My favourite isolated cannabis factoid is that the British Indian Civil Service compiled a seven volume, 3,281 page report on “Indian hemp” as far back as 1894.

    Here is a quote:

    “Mental Effects

    In respect to the alleged mental effects of the drugs, the Commission have come to the conclusion that the moderate use of hemp drugs produces no injurious effects on the mind. It may indeed be accepted that in the case of specially marked neurotic diathesis, even the moderate use may produce mental injury. For the slightest mental stimulation or excitement may have that effect in such cases. But putting aside these quite exceptional cases, the moderate use of these drugs produces no mental injury. It is otherwise with the excessive use. Excessive use indicates and intensifies mental instability (1:264).”

    Any comments from the pyschiatrists hereabouts as to whether we have notably changed this view (scientifically) in the last 113 years?

  4. stever said,

    July 31, 2007 at 10:30 pm

    more crap reporting of today’s cannabis worse tahn tobacco paper in Thorax…

  5. jdehls said,

    August 1, 2007 at 8:40 am

    I feal sorry for left handed tokers after reading this story:

  6. Dr Aust said,

    August 1, 2007 at 11:47 am

    There are some sensible letters in the Guardian today:,,2138349,00.html

    – forming a sort of small oasis of sanity in a desert of fatuous pontificating.

  7. banshee said,

    August 1, 2007 at 2:36 pm

    Hi Dr Aust – I’m not a psychiatrist but I do work in that arena!

    Trouble is the definitions and diagnoses change and I don’t recognise some of the terms used in the Indian Civil Service article! (neurotic diathesesis? Sheer phlogiston!)

    Would you take a century plus-old review of the harms of hemp especially seriously – where were the RCT’s?


    But they do appear to be hitting the nail on the head “A little bit of what you fancy….” etc.

  8. bludwulf said,

    August 1, 2007 at 9:20 pm

    well it seems Dr Aust beat me to it , but I wanted to comment on the use of cannabis to self medicate for stress/anxiety problems.
    I must say from personal experience that ranks right up there with “I just like the way it makes me feel” as reasons I’ve heard for smoking a lil grass.

    Long time reader first time poster
    I love your work Ben,
    Keep it up!

  9. Dr Aust said,

    August 2, 2007 at 2:22 pm


    Obviously a lot of the terminology is different. The counter-point is that they were probably the last (only?) survey of this ever done by an official without it being a football of some perceived moral agenda or agendas.

    It would be hard to argue with the view that, since Anslinger’s and others’ drug crusade in the States in the 20s and 30s, it has been pretty much impossible to have any sort of reasoned public or political discussion of “drugs of abuse” in general, and cannabis in particular.

    So partly the question to those with professional experience in the “substances” field was (rephrased slightly): if we replace the outdated terminology of the 1894 Indian Civil service report with modern versions of the same concepts, how well do we feel the conclusions stand up?

    I have had a go below, though as I am not a medical doctor, let alone a psychiatrist, the terminology is likely wrong. Corrections welcomed.

    “In respect to the alleged mental effects of the drugs, the Commission have come to the conclusion that the moderate use of hemp drugs [i]does not cause or exacerbate mental difficulties[/i]. It may indeed be accepted that in the case of people with [i]more severe problems within the neurotic disorder spectrum[/i], even moderate [i]regular[/i] [cannabis] use [i]has the potential to cause exacerbation, [since] regular use of any mind-altering substance (e.g. alcohol) has the potential to exacerbate underlying problems in such relatively vulnerable individuals[/i]. But putting aside these quite exceptional cases, the moderate use of these drugs does not [i]damage mental health[/i]. Excessive use [is different] since [i]this is more likely to show co-morbidity with mental disorders.”[/i]

  10. stever said,

    August 2, 2007 at 5:19 pm

    erowid will tell you all you can possibly want to know and lots more besides.

  11. Gimpy said,

    August 3, 2007 at 8:17 pm

    71. cheers. interesting.

  12. scientia said,

    August 5, 2007 at 2:07 pm

    If anybody is still reading this.

    The real elephant in the room is methamphetamine. There is ample literature that suggests the use of methamphetamine produces schizophrenia-like symptoms.

    Surely it is reasonable to believe that many, if not most, crystal meth users would also use cannabis..

    I would not be surprised if it was the cannabis users who also use metamphetamine that are responsible for the figures suggesting a link between cannabis and psychosis.

    In my opinion, the scaremongering headlines do not establish whether it is cannabis itself or the user of other drugs which contribute to the development of psychosis.

  13. RS said,

    August 5, 2007 at 7:15 pm

    scientia – at least some of the studies have attempted to control for use of other drugs such as amphetamine and cocaine.

  14. richard_p_auckland said,

    August 7, 2007 at 5:26 am

    I’d think it might be hard to control for other drugs, given that people would possibly be willing to be upfront about a widely accepted drug like cannabis, but not about a demonised (in NZ) substance like methamphetamine.

    Though I seem to remember from Stats 101 that there’s a way around this – something about asking the subject to tick a box if they’ve either committed arson or they have a living grandmother. Since the latter can be predicted, you can extract the figure for the former.

  15. atavachron said,

    August 12, 2007 at 12:21 am

    I had a number of ‘turns’ while smoking skunk – waking up in a state of abject undirected terror and dread. Then I stopped for a bit and it didn’t happen again – for a while. I think my own ‘psychosis’ had much more to do with my life being in the shitter than with how much THC I had in my blood stream. All anecdotal, carry on….

  16. hass said,

    December 19, 2007 at 12:36 pm

    man after reading all this it really made me light anthor joint..haha…i dont see why people shuldnt smoke cannabis i think the law shuld let it free and repect the one who smokes and the one who dont. thats my oppinion as a matter a fact am stonded right now thats why i find it real funny hahaha..and heres a shout for all those who smokes budzz “GOD BLESS YOU”

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