Cannabis psychosis – more reefer madness or not?

Children and young teenagers are best advised not to use drugs of any kind, not just cannabis. Young brains grow as they develop – a process termed “neuroplasticity” –  and drugs taken at a young age are bound to interfere with that process. Because of this, regardless of the need for evidence of harm, we should do what we can to prevent young people from accessing all recreational drugs and to delay the age of first use as long as possible. That is one of the strongest arguments against prohibition and in favour of cannabis law reform, it is not an argument in favour of continued prohibition which has seen the age of first use drop steadily over the years. Sadly the way this is usually reported by the tabloid press is far from objective, and indeed this lack of objectivity isn’t confined to the tabloid press.

The Today programme on Radio 4 is hardly to be compared with the lower reaches of the gutter press – like the Daily Mail for example – but this morning’s report about a new study went some way to bridging that gap. The study in question is:

Continued cannabis use and risk of incidence and persistence of psychotic symptoms: 10 year follow-up cohort study  (read it here)

The BBC coverage on today was quite clear in its message  (listen / download MP3 file here); the study and published conclusions were reported without any critical analysis which is par for the course when it comes to cannabis reporting. But toward the end of the item the studio presenter, Sarah Montague, asked the reporter, Danny shaw, what this means for the cannabis debate and how this plays into the debate about the classification of cannabis.  Danny Shaw made the following statement:

I think this adds support for the government’s – the Labour government’s – stance that it took three years ago to upgrade cannabis from class C to B. That was against the recommendation from the Advisory council on the misuse of drugs which said at the time there was a probable but weak causal link between psychotic illness and cannabis use and that cannabis played only a modest role in the development of these conditions.  It’s possible that the advisory council may have to review its findings in the light of  this research which is one of a series of studies which is really emphasising this very strong link between cannabis and psychosis.

As for the gutter press – for example the Daily Mail – their coverage was as expected, the story, headlined Cannabis use ‘doubles risk of psychosis for teenagers‘ covered all the claims of an increase in psychosis caused by cannabis and included an interesting quote from Professor (now Sir) Robin Murray

In short, it adds a further brick to the wall of evidence showing that use of traditional cannabis is a contributory cause of psychoses like schizophrenia

He didn’t explain what he meant by “traditional cannabis”, nor how this study shows any such thing and in fairness there is somewhat less than a “wall” of evidence to support the claim that cannabis actually causes serious mental illness.

In passing it’s interesting to note the Mail also emphasised the reefer madness scare by accompanying this report with a side bar story headed “Killers addiction”

Jake Fahri had been using  cannabis  since he was 12 before slashing another teenager’s throat  in ‘three minutes of absolute madness’ after a trivial row in a baker’s shop.

But enough of the Daily Mail, we expect nothing less.

The quote from Robin Murray was widely reported across the media (google search), it is perhaps a classic reefer madness soundbite of the sort the press love. So from the media’s coverage of this  can we assume the authors are supportive of prohibition? Well, it seems not. Somewhat less well reported in the UK were comments from Professor Wayne Hall from the University of Queensland and Professor Louisa Degenhardt from the Burnet Institute in Melbourne, as Science Daily reported

The major challenge is to deter enough young people from using cannabis so that the prevalence of psychosis is reduced, say experts from Australia in an accompanying editorial.

Professor Wayne Hall from the University of Queensland and Professor Louisa Degenhardt from the Burnet Institute in Melbourne, question the UK’s decision to retain criminal penalties for cannabis use, despite evidence that removing such penalties has little or no detectable effect on rates of use. They believe that an informed cannabis policy “should be based not only on the harms caused by cannabis use, but also on the harms caused by social policies that attempt to discourage its use, such as criminal penalties for possession and use.”

This, of course, throws a totally different light on the debate which is far different to the argument presented by the BBC or most of the British media and supports the statement made in the opening paragraph of this blog. In short, if cannabis is dangerous, it should be properly controlled and the trade properly regulated. So why is it that in this country the agenda is set, no debate is ever allowed and prohibition almost never questioned in the mainstream media?

The study claims to demonstrate that continued cannabis use increased the risk of a psychotic incident by nearly double. Of course, this is double a small number so it’s not a huge risk, but psychosis is a nasty illness and it does affect young people more than or adults, so clearly we do need to take some measures to protect this vulnerable group.

But is the study actually valid in its conclusions?

Imagine if a study were done into the effects of drinking and the study made no attempt to quantify what type of alcohol was being drunk – whether it was vodka or beer – and under a regime where that booze had been supplied by an unregulated illegal trade and was a highly uncertain quality product. Would anyone in their right minds draw any meaningful conclusions from such a study about the dangers of alcohol as supplied by a regulated and properly controlled trade? Of course not.

In addition to the complications created by an uncontrolled trade supplying a polluted product of unknown strength, cannabis presents another, additional unknown as a prohibited drug. Cannabis comes in many forms, containing different ratios of active chemicals (especially THC and CBD) which is thought to have a strong bearing on the mental health debate. This study made no attempt to identify the type of cannabis being used and in all honesty it couldn’t do so because of the regime of prohibition.  Worse it made no mention of these complications.

It’s also very likely the cannabis users studied smoked their cannabis with tobacco, it’s not at all clear if the study made any attempt to look only at users of pure cannabis but the word tobacco occurs only once in the paper’s text and that’s when describing the “Munich composite international diagnostic interview (DIA-X/M-CIDI)” technique. Controlling for “drugs” appears to mean

The variable use of other drugs included psychostimulants, sedatives, opiates, cocaine, phencyclidine, and psychedelic drugs

Likewise alcohol only features in the description of CIDI.

Not controlling for tobacco use is a major flaw however, because cannabis used with tobacco is often smoked to relieve a tobacco craving rather than the cannabis effect, thus leading to higher levels of use. People with psychotic illness are known to smoke tobacco at a far greater rate than people without the illness. Tobacco is not a neutral benign drug in this debate, so why wasn’t this complication even acknowledged?

The study also makes the conclusion that cannabis use comes before the onset of the illness and therefore the self medication theory is not supported. This is rather a big assumption as people developing mental illness often show “pre-morbid” indications of the illness; symptoms of an illness that hasn’t shown itself yet. An example might be finding a meaningless association of words funny, or disruptive behaviour at school. It’s never easy to spot these symptoms and indeed, they  may not be symptoms if they are seen. But self medicating cannabis use, especially unusually heavy cannabis use, caused by the developing but as yet not apparent illness may indeed be a possibility.

These are glaring errors with this study which undermines the credibility of it to a large extent. The only conclusion that can safely be drawn is surely that cannabis with tobacco used under a regime of prohibition increases the risk of psychosis for young people.

But considerations like this count for nothing with reefer madness style reporting.


UKCIA is a cannabis law reform site dedicated to ending the prohibition of cannabis. As an illegal drug, cannabis is not a controlled substance - it varies greatly in strength and purity, it's sold by unaccountable people from unknown venues with no over sight by the authorities. There is no recourse to the law for users and the most vulnerable are therefore placed at the greatest risk. There can be no measures such as age limits on sales and no way to properly monitor or study the trade, let alone introduce proper regulation. Cannabis must be legalised, as an illegal substance it is very dangerous to the users and society at large.

19 thoughts on “Cannabis psychosis – more reefer madness or not?

  1. Some good analysis here Derek – maybe post a rapid response (stick to the criticsm of the papers methodology though).

    I actually think that the paper is pretty good compared to many we have seen, but has been poorly reported as you point out – but the overlooking of the editorial that accompanied it is particularly galling.

    That said – suggetsing that cannabis use can increase risk of pshychotic symptoms (particularly in young people / vulnerable inndividuals – and is dose dependent) is neither new nor suprising.

    It feels rather like one of the intermittant ‘drugs bad for you – SHOCK’ stories that accompany almost any drug research, rather like the ‘loads of people taking drugs SHOCK’ coverage that you get following annual prevalence surveys. As ever, people will take from it what they want.

  2. Yes, more propaganda to try and back up camerons claim about highly damaging and very very toxic cannabis.

  3. I used to have the same relaxed attitude towards cannabis and even tried it myself to find out what all the fuss was about whilst in my late teens.
    Maybe the goverment are not overreacting especially as young people are now experimenting at an earlier age and this is leading to an increase in Psychotic illness.
    I post this as a parent who has a step son with drug indused Psycosis and paronoid schizophrenia. I also work in a hostel and see these effects regularly from cannabis users.

  4. Bird bantam, with repsect I don’t think this blog can be accused of having a “relaxed attitude” towards cannabis – the very first paragraph talks about the risk of recreational drug use.

    The need to restrict the avaiablity of drugs including cannabis to young people is not in doubt, which is why I wrote “we should do what we can to prevent young people from accessing all recreational drugs and to delay the age of first use as long as possible”. Prohibition, I’m afraid, simply does not do that, as you yourself have seen.

    The main point of this blog though is I think pretty clear; this study doesn’t prove anything about cannabis as such, if it shows anything at all it is what is happening under the regime of prohibition.

  5. ‘Increasing Doubts’

    Hi UKCIA,

    Firstly, media coverage of substance issues is often ill-informed, hysterical and geared towards popular impact and sales. So don’t trust it, it’s biased.

    Secondly, politicians feed into this because they need the mass vote to get re-elected, so they can’t be seen to go against popular opinion. Also, international agreements leave their hands quite tied, and they need credibility with international partners across a range of more important issues, meaning that cannabis is not a sensible area for them to choose a battle with significant allies. So don’t trust them, they’re biased.

    Thirdly, our legislation is a mess, as outlined by the recent report by David Nutt. Dangers and risks are not addressed equitably by current or historic legal processes.

    Fourthly, what I’d be interested to know about you, though, in the interest of credibility and balance, is if you have ever run a report on research or media coverage that revealed or highlighted dangers of cannabis use, or demonstrated academically verified evidence, in which you did NOT question its credibility?

    Are you so set in your opinion, that you are programmed to see any evidenced contrary opinion or research as “glaring errors with this study which undermines the credibility of it to a large extent”?

    As night follows day I could already FEEL your response coming, as soon as this research was publicised yesterday morning in the Drugscope news roundup … and lo – here it is the next day.

    If you never allow a study to come out without a kneejerk analysis that writes it off within 24 hours, and tries to undermine its credibility or carefully select a quote where the wording was not quite perfect, then perhaps your judgement, like the media’s, might also not be completely without prejudice.

    Or are you also playing to the popular opinions of your own audience? Are you, in fact The Daily Mail for cannabis proponents?

    Can you show that you have ever published a favourable analysis of a report or research that lead to a conclusion you did not hold before it was published?

    Do we need to come to the same conclusion about you – Don’t trust them, they’re biased? Or can you show genuine objectivity and allow your opinions to be changed by the evidence, and advise your readers accordingly?

    (NB: I’m referring to the News responses section of your output, not the advice sections such as:
    Even so, I would argue that it may not be totally accurate to state that the links between Cannabis and MH didn’t hit the headlines until 2004.)

  6. Hi N.Q.Irritatus

    Can you show that you have ever published a favourable analysis of a report or research that lead to a conclusion you did not hold before it was published?

    An interesting question.

    You might like to look at the reviews of the Cannabis and mental health conferences from 2004 and 2007 for a start

    I’ve even been nice about the generally awful “Talk to Frank” when they do things right

    Thing is science has some fundamental principals which cannot be compromised; In order to do science you do need to be able to measure the thing you’re studying and there are established ways to collect data. Any study which uses data collected by unrepresentative sampling methods is based on garbage data and the old maxim is always true: Garbage in/garbage out, it doesn’t matter how it’s dressed up to sound authoritative.

    Also papers should always highlight shortcomings in data collection, problems with confounding factors and so on. Again, that is an obligation, not an option.

    I make no apologies for criticising scientific studies which fall short of these standards and it is true that so many studies into cannabis do. Prohibition is like that, it prevents proper data collection therefore it prevents proper science and any paper which ignores that truth is really trying to pull a fast one in my opinion.

    Take for example the Home Office potency study as a few years ago
    An extreme example of cod science maybe, but it was used as a “factual” basis for much media hype and the extension of the criminal law.

    This site is also critical of efforts which seek to promote law reform where there is reason to do so, an example of that would be the critique of the Protest London efforts a while back

    I do try to be objective, but the role of this site is to be critical of the claims made by politicians and anyone else, and that includes scientists.

  7. Hmmmmmm,

    Having looked at the examples you quote:

    … there’s no evidence here of an example of UKCIA learning something new and changing its position.

    These are simply examples of agreeing with evidence for the position you already held.

    I agree that bad science needs challenging, and ill conceived protests need criticising, even when the protest is legitimate. These examples are the UKCIA performing a good service.

    My point is this. You can’t simply decide that all research is invalid unless it backs up your position.

    In order for the UKCIA to have the credibility needed to maintain a sensible campaign, you need to be able to show respect for academically verified evidence, rather than simply question the credibility of all research that comes to a different conclusion than your existing position.

    Hence it may be worth looking harder at the edges of the research and finding conclusions you hadn’t arrived at, endorsing and communicating them even if it means a change in your position, and seeing if you start to be taken more seriously in the debate.

    NB: The study you are critiquing this time seems very robust. Academic and Health research via a Dutch University, 10 years, 4 countries, nearly 2,000 individuals, peer reviewed before publication, accepted in Dec 2010, and then published in the BMJ in March 2011.

    Is that ‘Garbage in’ or simply ‘Awkward result out’?

  8. N.Q.Irritatus, The idea of neuroplasticity I mentioned in this blog was a new concept obtained from the conferences, as was the argument about THC/CBD ratios.

    I do accept your point that even if it hurts, the truth is the truth.

    I should also add that where possible I always reference the original paper, encourage people to read it and provide a forum for comments.

    My point is this. You can’t simply decide that all research is invalid unless it backs up your position.

    Absolutely not and I hope it doesn’t come across that way, if it does apologies. But the rule still applies; to make a conclusion from a study the “rules” of good science must be followed by the study and the extent of the conclusions drawn should be governed by the limitations of the study.

    You say this paper seems robust, I would dissagree obviously. Take for example the issue of tobacco use. From my reading of the paper (and I will be happy to be corrected) no consideration at all was given to what is clearly a huge confounding factor. Was the observed effect due to the cannabis, or the combined effect of both drugs?

    There are other variables not considered by the study; one example might be whether the affected people were using cannabis grown intensively using large amounts of organo-phosphate pesticides? I don’t know what effect smoking pesticide residue might be on mental illness, or even to what extent this might be a problem, but again we might be seeing an effect caused not by cannabis but the political regime it exists under. Pesticide conamination is just one of the “unknown unknowns” created by prohibition.

    If this study can be replicated under a regime whereby people are using pure unadulterated cannabis of consistent quality then the observed results can fairly be put down to cannabis use. As it is, the results can only be attributed to cannabis use with tobacco under a regime of prohibition.

    I do wonder why other respected academics haven’t picked up on such obvious shortcomings which I hope you accept are real?

  9. Yes, these are good questions and comments, but ones which may be defensible if the researchers were actually drawn into a dialogue, rather than immediately publicly criticised.

    Do you want good research on this subject? If so, an immediate predictable rebuff doesn’t encourage researchers to want to enter this field, or maintain an open dialogue when they may fear their credibility or professionalism will be questioned.

    Why not in future quickly flag up this type of research for people’s attention, but with minimal immediate comment.

    Then invite observations from readers, and then try to facilitate a dialogue with the source or publisher of the research.

    [e.g. In this case the email address of the researcher is on the full report, it’s:

    You said “I do wonder why other respected academics haven’t picked up on such obvious shortcomings which I hope you accept are real?”

    Did you ask them or the BMJ for clarification? It must have been peer reviewed.]

    After that, then release a considered response after a dialogue has at least been attempted.

    This would then look less like a publicity/media battle, less like UKCIA preconceived kneejerk reactions, and more like a genuine dialogue to find out the truth and apply any lessons learned.

    Again, I think that UKCIA credibility would improve, as may leverage in an important debate, or even the ability to sponsor or suggest areas that would benefit from research.

    As I said before, at the moment when a report or research comes out with a negative slant, you can predict the timing, style and nature of the UKCIA response at once.

    I think you need to find a way not to simply appear to mirror the bad science and bad reporting that obviously frustrates and irritates you into this pattern.

  10. Thanks for the comments N.Q.Irritatus, it’s an interesting situation we find ourselves in.

    The problem is we are faced with a massive media onslaught, whereby the results of these studies are used by politicians and the media to justify the existence of the criminal law. Sadly this isn’t just an academic debate; it is a very public social/political issue.

    I will take this up in some way with the BMJ, but the damage has already been done in that the media has been able to use this study as further “proof” that cannabis makes people “go mad” to use Daily Mail speak.

    As I said before, at the moment when a report or research comes out with a negative slant, you can predict the timing, style and nature of the UKCIA response at once.

    If that is true it’s probably because the same flaws exist in the studies and in truth, all too often they do.

    I think you need to find a way not to simply appear to mirror the bad science and bad reporting that obviously frustrates and irritates you into this pattern.

    The hope I have with this website is that it will motivate people to question what they are told as fact. But you’re right; I am frustrated and angered by the level of debate in this country.

    It’s interesting that you haven’t actually challenged any of the substantive points I’ve raised in criticism of this study incidentally.

  11. On the one hand:

    My specialism within substance misuse is alcohol.

    I agree with David Nutt’s analysis.

    The hypocrisy of our current legal and societal position on classification is an accident of social history that is not lost on me.

    I include myself in the hypocrisy – I drink alcohol, allbeit sensibly.

    On the other hand:

    Although cannabis is arguably more safe than many other substances, even legal ones, for over a decade I have seen the CORRELATION (causal connection or direction NOT proven) between cannabis use and mental health issues in people I have tried to help in my working life and among friends outside of my work.

    I would still advise anyone with family history in depression or mental health issues to avoid cannabis to be on the safe side, at least until the evidence is more clearly understood and conclusions more universally agreed.

    I don’t claim to be an expert on the specifics that flaw this research for you. (Hence not feeling qualified for a moment to disagree with your expert opinions or criticisms of the research.)

    I wish we were in a context where a more mature dialogue was possible, rather than in such a volatile political and cultural crucible.

    The only way forward must be to model the quality of engagement that is needed from all sides. Even so, this will make no difference overnight, or even for a long time.

    In the meantime, thanks for the quality of THIS dialogue!

  12. What blows my mind is that people return to the issue of causal or correlative links to mental health issues when we openly permit activities with proven causal relations to death!

  13. i read a long time ago (8 years ish) that there is a link between alcohol abuse and scitzofrenia, and that it was widely accepted. (but) this link was that people with the condition almoste always were drinking loads, smoking loads and doing pretty much wot they could to be off there head and esacape there condition.
    since then i have largly regarded all the “reffer madness” as people playig off the scitzofrenics tendencey to abuse all drugs not just alcohol or cannabis. and pinning it on cannabis as its ilegal and needs justification beyond “because it is”.

    im yet to see a study of weather alcohol can also make people “mad”. i wonder if given to an objective or negative view of alcohol, those people might say the same as we(the uk) are about cannabs in view of weather there is a link or not.
    in this coutry pertymuch every one loves booze so are more redy to accept that its “just skitzos getting off there head” and that they are in no dager of it happening to them.

    though i have herd of “alcohol demetcha”. witch no one ever told me at scholl or on the tv or any of the drugs advice i have had forcevly rammed down my neck since i was 10. but i see it in the faces of the people i sell booze to every morning in the supermaket i work in and in the faces of the people stumbling around at 4 in the morning shouting random insult at lamp posts. i thought that was just what happened when peopl get old and drink too much but i have since come to a different conclusion.

    if only jesus smoked a spliff at the last supper? what a different world we might live in!

    (sorry for spelling and grammer im dyslexic and theres no spell check lol)

  14. What’s that famous old saying? Oh yeah: “there’s lies, damned lies, and most of the mass media”. Something like that.

  15. The experience of using controlled drugs is corrupted on so many levels; firstly there is the well-documented presence of contaminants and unknown strengths in drugs sold without consumer protection. The unknowingness of what users buy also can impact upon them psychologically. Cannabis for example does increase sensitivity and can lead to anxiety and neurotic behaviour in certain contexts. Context with certain mind-altering drugs is of paramount importance, perhaps analogous to a hypnotic subject, the experience can be used to benefit the person or in a different arena could be used to exploit or even harm the subject. As soon as cannabis users buy their drug they are prone to anxiety because of the fear of consequences of arrest, couple this with the heightened sensitivity of actually using it and for many people the various neurotic fears of opprobrium, surveillance, arrest etc can cause anxiety and neurotic behaviour, and eventually contribute to a psychosis. As soon as a person enters that space of being stoned, they are labelled as occupying a restricted zone, an off-limits mode of consciousness. The drug experience in the context of a perfection of harmony could for many users bring about well-being, spiritual awakening, euphoria or whatever expectations are front-loaded into the psyche. Cannabis increases what is there already and that is based upon expectation. Create an expectation of harm and a context of mistrust and eventually you will achieve it – the drug and the context are inextricably linked. You simply cannot assess the harmfulness of cannabis objectively whilst it’s use is demonised and a war declared on it’s users.

    I accept there is evidence about the harms caused by cannabis; my point is that it’s wishfully neutral but fails on numerous flaws to the methodology. It would be like saying when abortion was illegal, that the reason to maintain that policy was there was evidence of the practice causing harm to women undergoing the procedure – I don’t need to point out the irony of this any further. There may be a pocket of concern after we have stripped away the whole criminal context of the cannabis experience, but we cannot see it for it does not exist in a vacuum. If it exists, the way to deal with it is through sensible advice on consumption type, amount and frequency or abstinence (such measures being proportionate and relative to those applied to other drug users such as drinkers).

  16. This from NHS choices..pretty balanced( well nearly)

    Young people who use cannabis are doubling their risk of developing psychotic symptoms,” the Daily Mail has reported. Mental health problems may also persist among continual users, it added.

    The news is based on a study involving nearly 2,000 German adolescents and young adults. It found that new cannabis use almost doubled the risk of psychotic symptoms in the years after use. The study also found that these users had been free from psychotic symptoms prior to smoking cannabis. Previously, it has not been clear whether cannabis use leads to psychotic symptoms or whether young people with psychotic symptoms use cannabis to “self-medicate”.

    It should be noted that self-reported psychotic symptoms were assessed rather than clinically-diagnosed psychotic problems. Psychotic symptoms are not uncommon in the general population. Overall though, this large, well-designed study supports the results of previous research on the matter, suggesting there is an association between cannabis use and psychotic symptoms.

    Where did the story come from?
    The study was carried out by researchers from Maastricht University, Netherlands; the University of London, UK; the University of Basel, Switzerland; the Max Plank Institute of Psychiatry, Germany, and Technical University Dresden, Germany. The study was funded by the German government and was published in the peer-reviewed British Medical Journal.

    The study was generally reported accurately in the papers. The Daily Telegraph reported comments from external experts, one of whom pointed out that the study did not distinguish between different types of cannabis. However, some newspapers featured the specific claim that cannabis use doubles the risk of psychosis, which could be considered to be inaccurate as the study found that using cannabis doubled the risk of self-reported psychotic symptoms rather than clinically-diagnosed psychotic illness. None of the papers pointed out that the study relied on young people recalling psychotic symptoms, rather than on clinical diagnosis.

    What kind of research was this?
    This was a prospective cohort study of nearly 2,000 German adolescents and young people who were followed-up for a period of 10 years, to determine whether use of cannabis in adolescence increases the risk of “subclinical” psychotic symptoms (i.e. symptoms below the level required for a clinical diagnosis). It looked at both ‘incident’ (i.e. new) psychotic symptoms in cannabis users and non-users. It also looked at whether psychotic symptoms persisted in those who used cannabis.

    The authors say that cannabis use is already associated with an increased risk of psychotic disorders. However, it is not known whether cannabis use itself increases the risk or if the association is due to people with pre-existing psychotic symptoms tending to use cannabis as a form of “self-medication”. Also, the mechanism by which cannabis use might cause psychotic symptoms is not yet understood. In this longitudinal study the researchers set out to look at first-time cannabis use in relation to first-time psychotic symptoms.

    What did the research involve?
    This was a population-based cohort study that took place in Germany, involving 1,923 participants from the general population. The participants were aged between 14 and 24 at the start of the study. The sample had been drawn from an earlier study, which collected data on mental disorders in a random sample of adolescents and young adults.

    The researchers collected information from participants on both cannabis use and “subthreshold” psychotic symptoms at the start of the study (baseline). They also collected information at three further points in time: on average, these were 1.6 years (T1), 3.5 years (T2) and 8.4 years (T3) after the study began. In collecting information, they used a validated diagnostic interview which assesses symptoms, syndromes and diagnoses of various mental disorders in accordance with internationally agreed definitions.

    The interviews were conducted by trained clinical psychologists. The diagnostic interview also included questions about substance use. The presence of psychotic experiences, as defined by the diagnostic interview, included symptoms such as delusions, hallucinations, feelings of persecution and thought interference.

    Within the same interview, participants were also asked if they had used cannabis five times or more. Cannabis use on at least five occasions was used to define cannabis exposure, which was recorded as either “yes” or “no”.

    The researchers then used standard statistical methods to assess the relationship between cannabis use and both new and persistent psychotic symptoms. The results were adjusted for “confounders” that might have influenced results, such as sex, age, social and economic status and other drug use. However, researchers were unable to take account of family history of psychosis, which could have influenced the risk of psychotic symptoms.

    What were the basic results?
    They found that:

    In young people who had not reported psychotic symptoms or cannabis use at baseline, starting to use cannabis between baseline and the T2 phase increased the risk of later having new (incident) psychotic symptoms over the period from T2 to T3 (adjusted odds ratio [AOR] 1.9, 95% confidence interval 1.1 to 3.1).
    Continued use of cannabis increased the risk of persistent psychotic symptoms over the period from T2 to T3 (AOR 2.2, 95% CI 1.2 to 4.2).
    31% (152) of people who had been exposed to cannabis reported psychotic symptoms over the period from baseline to T2, compared to 20% (284) of individuals who had not been exposed.
    Over the period from T2 to T3, 14% (108) of those exposed to cannabis reported psychotic symptoms, compared to 8% of the participants who were not exposed.

    How did the researchers interpret the results?
    The researchers say their study has found that cannabis use is a risk factor for the development of incident (i.e. new) psychotic symptoms, and that cannabis use precedes the onset of psychotic symptoms. They also say that continued cannabis use increased the risk of symptoms persisting. Therefore it might increase the risk of psychotic disorder.

    The researchers also suggest that the increased risk seen with cannabis use may be due to repeated exposure to THC (the main psychoactive component of cannabis), although there is currently a lack of evidence for this in humans.

    This large, well-designed study suggests that new cannabis use carries a risk of later psychotic symptoms in young people who had not previously had psychotic symptoms. It also suggests that continuing cannabis use might lead to persistent psychotic symptoms and that this could increase the risk of developing psychotic illness. However, the study had several limitations, some of which the authors have noted:

    It relied on self-reported information about both psychotic symptoms and cannabis use. This could potentially introduce error, although the authors say this possibility was minimised through their interviews being conducted by trained clinical psychologists.
    The study did not adjust for family history of psychosis, a possible confounding factor. The authors say they may have indirectly adjusted for this to some degree, though.
    The authors say they used a “broad outcome measure” to represent psychotic experiences, rather than clinically relevant psychotic disorder. However, they say that psychotic experiences show “continuity” with psychotic disorders.
    The study may have had its results influenced by “selective recall” about cannabis use and psychotic symptoms, i.e. the participants may have intentionally or unintentionally modified their answers to support their personal views on the matter. The long-term nature of this study may increase the risk of this occurring as the participants would have known the purpose and methods of the study and could have modified their answers at later interviews.
    In conclusion, this study’s findings are a valuable addition to the research on possible association between cannabis use and psychotic symptoms, particularly because it was able to show that use of cannabis preceded psychotic symptoms. However, further research is required into any association between use of cannabis and more long lasting, clinically diagnosed psychotic disorders.

    Links To The Headlines
    Cannabis ‘doubles risk of psychotic episodes’. The Daily Telegraph, March 2 2011

    Cannabis use ‘doubles risk of psychosis for teenagers’. Daily Mail, March 2 2011

    Cannabis ‘raises psychosis risk’. BBC News, March 2 2011

    Links To Science
    Kuepper R, van Os J, Lieb R et al. Continued cannabis use and risk of incidence and persistence of psychotic symptoms: 10 year follow-up cohort study. BMJ 2011; 342:d738

  17. I will first note that Derek in reviewing this supposedly COMPREHENSIVE study found almost no reference to tobacco in it– yet if a single drug is kill 6 million human beings a year, you’d think the presence of that drug (and its synergies with cannabis) was clearly the most important subject for any study of cannabis impact in such an environment? With Derek one must ask WHY no tobacckgo component to this and so many “cannabis” studies.

    As NQ said, “Politicians… need the mass vote to get reelected, so they can’t be seen to go against popular opinion.” The Dicky
    Trick here is that “popular opinion” on anything inhaled is just about exactly what $igarette companies spent a Trillion Dollars over the last century “advertising” it into being. (No one howwever wants to admit how dumbozilic we’ve all been had, the very TOLERANCE of this lethal hot burning torchery mythology is itself the #1 P.R. Shame of the last century and everyone just wants to forget their own cowardice.)

    Evidently not just politicians but RESEARCHERS– who are dependent on approval, funding, tenure, credentials– are cowed by this engineered “popular opinion” into ducking the tobacckgo issue whenever they discuss “drugs”.

    HOW Big 2WackGo terrorizes professors? I suspect largely in unconscious ways. Maybe you won’t find much overt evidence of pressure on any one writer. Any “immediate predictable rebuff” as N.Q. says. Here one may suspect something deliberately clandestine– as in the Dick Nicotine era, an agent named Donald H. Segretti ($igaretti?) was in charge of the black bag jobs (Dirty Tricks). Maybe they tip off some friendly Daily Mail reporter who invents a neat needle of ridicule against a researcher careless enough to say something pro-cannabis.

    Thus tobacckgo companies have been able to enslave and kill to an unprecedented degree in the history of the planet while succeeding in blaming a COMPETITIVE herb, cannabis, for much of the carnage.

    Remember– until a decade or two ago– and till today in third world settings– the tobacckgo advertising was so ubiquitous, Mother could be reading some magazine and the 2-year-old would think she was reading a story about a cowboy with a $igarette– because 2-year-olds don’t know the difference between “front” and “back” (tobacckgo) covers of the magazine. That’s what creates the largely unconscious “popular opinion”, and the “set and setting” rightly mentioned by Sunshine, in which expectation of harm helps achieve it– some wag named this the SELF-FULFILLING FALLACY.

  18. “NB: The study you are critiquing this time seems very robust. Academic and Health research via a Dutch University, 10 years, 4 countries, nearly 2,000 individuals, peer reviewed before publication, accepted in Dec 2010, and then published in the BMJ in March 2011. Is that ‘Garbage in’ or simply ‘Awkward result out’?”

    I made my own study of this study ( and I’m happy to dismiss it as garbage, frankly. Of particular relevance are i) Figure 3, in which the findings are summarised in a graph which in fact bears no relation to the results obtained: the longer you sit staring at that graph the more you see how absurd and propagandistic it is; and ii) this quote about how “The authors say they used a “broad outcome measure” to represent psychotic experiences, rather than clinically relevant psychotic disorder. However, they say that psychotic experiences show “continuity” with psychotic disorders,” well, you can go as deep as you like into that, you could point out for instance that the authors of this study casually mention at one point that the incidence of so-called “psychotic disorders” is estimated to occur within 15-28% of “the general population” i.e. this supposed sickness is a natural and common phenomenon, which “shows continuity” with actual psychosis in the same way that having a healthy appetite “shows continuity” with obesity and other eating-disorders.

  19. “…the incidence of so-called “psychotic *experiences*” is estimated to occur within 15-28% of “the general population,” that should have said.

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