Cannabis and mental illness – the Keele Study

Something we’ve been waiting for ages for has finally arrived; the government has (albeit very quietly) published the study from Keele University entitled “Assessing the impact of cannabis use on trends in diagnosed schizophrenia in the United Kingdom from 1996 to 2005”  (edit 2011 – read the full paper here)

Some 16 months back the Guardian ran an item about the proposed (as it was then) reclassification of cannabis to class B. Under the heading “Brown plans tougher line on cannabis against advice of own drug experts” the paper reported research carried out by a team from Keele University:

Their confidential paper found that between 1996 and 2005 there had been significant reductions in the incidence and prevalence of schizophrenia. From 2000 onwards there were also significant reductions in the prevalence of psychoses.

The authors say this data is “not consistent with the hypothesis that increasing cannabis use in earlier decades is associated with increasing schizophrenia or psychoses from the mid-1990s onwards.”

Note that last paragraph, we’ll come back to it shortly.

This was in April 2008, it is only now – on 26th June actually – the government has published this study and it hasn’t exactly gone out of its way to draw attention to it. UKCIA got wind of this not via the British media but from a tabloid style article from the US. The site published this story under a Sun style headline:

Chronic City: After Further Review, Smoking Pot Doesn’t Make You Crazy — Blimey!

After a while the style settles down a bit

For the new study, British investigators at Keele University Medical School compared trends in cannabis use and instances of schizophrenia in the United Kingdom from 1996 to 2005. The research showed that even as marijuana use soared among the general population, “incidence and prevalence of schizophrenia and psychoses were either stable or declining” during this period.

The authors concluded that an expected rise in diagnoses of schizophrenia and psychoses did not occur over the decade under study. “This study does not therefore support the … link between cannabis use and incidence of psychotic disorders,” the study concludes, adding “This concurs with other reports indicating that increases in population cannabis use have not been followed by increases in psychotic incidence.”

Notice again the conclusion  “This study does not therefore support the … link between cannabis use and incidence of psychotic disorders”. This seems to be a pretty well understood conclusion made by the Keele study.

A perhaps slightly more sober report came – again from the US – on the NORML blog. NORML is the American Legalise Cannabis Campaign known as the National Organisation for the reform of Marijuana  Laws. Paul Armentano, NORML Deputy Director wrote on July 1st:

… a team of researchers at the Keele University Medical School have once and for all put the ‘pot-and-mental illness’ claims to the test. Writing in a forthcoming edition of the scientific journal Schizophrenia Research, they compare long-term trends in marijuana use and incidences of schizophrenia and/or psychoses in the United Kingdom. And what do they find?

The NORML blog then appears to quote the abstract from the Keele study as saying:

“[T]he expected rise in diagnoses of schizophrenia and psychoses did not occur over a 10 year period. This study does not therefore support the specific causal link between cannabis use and incidence of psychotic disorders. … This concurs with other reports indicating that increases in population cannabis use have not been followed by increases in psychotic incidence.”

Strangely this conclusion isn’t included in the Pubmed abstract the blog links to and neither is it included on the original hosted by Schizophrenia Research, who are publishing the actual paper. Has it been changed? This seems very odd.

Schizophrenia Research publish this abstract (as at 4th July anyway):

Assessing the impact of cannabis use on trends in diagnosed schizophrenia in the United Kingdom from 1996 to 2005

Martin Frishera Corresponding Author, Ilana Cromeb, Orsolina Martinoa, Peter Croftc

Received 17 April 2009; received in revised form 27 May 2009; accepted 30 May 2009. published online 29 June 2009.
Corrected Proof


A recent systematic review concluded that cannabis use increases risk of psychotic outcomes independently of confounding and transient intoxication effects. Furthermore, a model of the association between cannabis use and schizophrenia indicated that the incidence and prevalence of schizophrenia would increase from 1990 onwards. The model is based on three factors:

a) increased relative risk of psychotic outcomes for frequent cannabis users compared to those who have never used cannabis between 1.8 and 3.1,

b) a substantial rise in UK cannabis use from the mid-1970s and

c) elevated risk of 20 years from first use of cannabis.

This paper investigates whether this has occurred in the UK by examining trends in the annual prevalence and incidence of schizophrenia and psychoses, as measured by diagnosed cases from 1996 to 2005. Retrospective analysis of the General Practice Research Database (GPRD) was conducted for 183 practices in England, Wales, Scotland and Northern Ireland. The study cohort comprised almost 600,000 patients each year, representing approximately 2.3% of the UK population aged 16 to 44.

Between 1996 and 2005 the incidence and prevalence of schizophrenia and psychoses were either stable or declining. Explanations other than a genuine stability or decline were considered, but appeared less plausible. In conclusion, this study did not find any evidence of increasing schizophrenia or psychoses in the general population from 1996 to 2005.

In the index list of publication in print there is a link to the full text, which asks for a registration log-in. If you create an account and get to the linked page, you see this message:

This article may not be purchased.
Access is limited to print subscribers.

So it appears they are really not going out of their way to make this study and especially it’s conclusion public.I suspect had the conclusion been different it would have been splashed all over the tabloid press.

Thing is, this is dynamite evidence against the claims of a causal role for cannabis in the development of psychosis and frankly kills Reefer madness V2.0 stone dead. Why? Because simply this study covers the decade  when both the both rates of cannabis use and the prevailance of so-called “skunk” were on the rise. If cannabis and especially the high THC versions had a causal role in the development of psychotic illness we would have seen an increase in those rates. Now, had the study found such an increase it would have supported although of course not proved the theory of a causal link between cannabis and mental illness. However, finding no increase (and indeed, perhaps even a decrease) provides a pretty definite conclusion: Cannabis does not cause psychotic illness.

Although it doesn’t actually say so in the abstract, the conclusion that “This study does not therefore support the specific causal link between cannabis use and incidence of psychotic disorders. … This concurs with other reports indicating that increases in population cannabis use have not been followed by increases in psychotic incidence.” is inescapably true.

If anyone can get hold of a copy of this study, UKCIA would dearly love to see it (edit – the paper is here).

Now this has a very important consequence for everyone in the UK, way beyond the single issue of cannabis and it’s ranking in the Misuse of drugs act. Our Prime Minister Mr Brown was made aware of this study before he made his decision to return cannabis to class B. As the Home Office put it to recently – as reported on this blog a few weeks ago:

The Frisher and Crome analysis of a fall of prevalence of diagnosed psychoses and fall of prevalence and annual incidence of diagnosed cases of schizophrenia and psychosis in general practice between 1996 and 2005, to which you refer in your letter, was reported by ACMD in its most recent report and was taken into account before coming to its conclusions on this issue.

So they saw the study and decided to ignore it.  So we now have evidence like never before that the Prime Minister not only ignore the advice of experts, which is bad enough, but he actually ignores hard evidence when it doesn’t fit with his own prejudices. We knew this before, now we have evidence. This is serious and deeply worrying for the well being of the country, after all, if he did it with this issue he could do it with any.

The item mentioned above also mentioned another study which for some reason seems not to have grabbed the media’s attention.

Can recreational doses of THC produce significant dopamine release in the human striatum?

Stokes PR, Mehta MA, Curran HV, Breen G, Grasby PM.

Psychiatry Group, MRC Clinical Sciences Centre, Imperial College, Hammersmith Hospital Campus, Du Cane Road, London, W12 0NN, UK.


Cannabis use in early adolescence may be a risk factor for development of schizophrenia. In animals, Delta9-tetrahydrocannabinol (THC) increases the rate of dopamine neuronal firing and release in the striatum. Thus cannabis use may increase dopamine release in the human striatum leading to vulnerability to psychosis.

AIMS: To investigate whether THC, the main psychoactive component of cannabis, can produce dopamine release in the human striatum.

METHODS: Thirteen healthy volunteers, with previous cannabis experience, underwent two [11C]-raclopride positron emission tomography (PET) scans to indirectly measure striatal dopamine levels following either 10 mg THC or placebo.

RESULTS: Although THC markedly increased psychosis-like symptoms on the Psychotomimetic States Inventory (PSI), there was no significant effect of THC on [11C]-raclopride binding.

CONCLUSION: In the largest study of its kind so far, we have shown that recreational cannabis users do not release significant amounts of dopamine from an oral THC dose equivalent to a standard cannabis cigarette. This result challenges current models of striatal dopamine release as the mechanism mediating cannabis as risk factor for schizophrenia.

All this makes the claim by the home Office position reproduced on the Talk to Frank website that

The regular use of cannabis is known to be associated with an increase in the risk of later developing psychotic illnesses including schizophrenia.

Look simply wrong and no more than government anti drug propaganda.

In conclusion  what this shows is that increased use of cannabis has not lead to an increase in the rates of the illness, so there can be no causal role for cannabis in the development of serious mental illness.This does not mean that unrestricted use by kids is OK, it’s not for a whole load of reasons to do with developing brains, learning ability and so on. It does mean the move back to B was done for the worst of all reasons – to satisfy the tabloid press and it does cast a doubt on the Prime Minister’s creditability.

It is also probably true that ill people are at risk from cannabis use and so need to be regarded as a vulnerable group deserving of special protection.


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.

24 thoughts on “Cannabis and mental illness – the Keele Study

  1. I always find it interesting that those who want legalisation find ways to make research say what they want. In your article it makes use of the words “Although it doesn’t actually say so in the abstract,” from this I wonder how authors came by further information ? telepathy maybe? the reports have no definite conclusions and some are based on population studies not on actual users,I wonder how many persons who had a psychotic episode never came to the attention of the Hospitals and GP’s because family, friends etc just coped with them or allowed them to be weird and hid it from the world in general. Therefore there appeared to be a reduction or no increase. None of this information should be splashed across the media or even used by sites like the above to prove what they want. It is pure research and should be viewed as such for tomorrow another reputable University/ research facility could come out with completely different results. My feeling is that if they did I would not read it on the above site

  2. >… another reputable University/ research facility could come out with completely different results. My feeling is that if they did I would not read it on the above site

    So, some unknown ‘conlcusions’ that cant be found ‘anywhere else’ and preemptively ‘not reported here’ is proof if your (faith in) scientific method? What humbug is this?

    The contest of ideas surrounding cannabis was reported in the body of and conclusions of an evidentiary review conducted in 1998 by a very circumspect New Zealand House of Representatives Health Select Committee “Cannabis and Mental Health” concluded “the harms are largely overstated”. The report was notable in its call for the law surrounding cannabis to be reviewed.

    Ten years later and absent any gnashing of teeth by politicians or media, NZ became the first country in the world to put in place ‘legal regulations’ for recreational ‘soft’ drugs, including place of sale, storage, labeling, packaging, advertising and age of consent administered under the aegis of the Ministry of Health. The recognition that ‘the prohibitory law’ is entirely unhelpful in mediating health concerns is obvious.

    It is unlikely you would find that pragmatism in any University study that depends on its funding coming from the treatment sector.

    But then you would have to come to this site to find that out….

    (check my blog and search for the UN convention compliant “Class D”)

  3. ‘Mickey Mouse’ questions the article on the grounds that the decline/stability of mental illness could be a result of under-reporting.

    Does Mickey have any evidence for this or is it just a grab in the dark from a prohibitionist?

    Seems to me the health system is far more onto diagnosis of these illnesses these days. But I would not offer that opinion in a critique of anything scientific because I have no evidence for it.

    Mickey could equally have speculated that the decrease/stability in mental illness — while cannabis use increased — was because cannabis reduces the risk of such diseases.

    But he didn’t and neither would I, because I have little evidence for it. Seems to me it is Mickey ‘trying to make research say what he wants’.

    I thought the article’s honesty about the apparently unsupported conclusions was a sign of balance and objectivity.

  4. After 40 years of watching wonderful talented people destroy themselves. As well as researching the reasons they do. I have often wondered where I fit in the scheme of things. My colleagues say I am an intuitive harm minimisationist. But now I see I am a blinkered prohibitionist who is akin to a mushroom. unfortunately neither sits comfortably with me, I am Mickey Mouse who has a very open mind and patience to wait until all facts are in to say categorically that something is fact.
    The topic that is under discussion is and I quote “This is a complex topic to investigate for several reasons. Firstly, it requires longitudinal data to ascertain causal sequences. Secondly, rates of substance misuse and psychiatric illness in general population studies tend to be low and thus power may be limited. Thirdly, it is necessary to consider potentially confounding factors and most extant studies are limited in their ability to do this……”(Martin Frisher1, Ilana Crome2, John Macleod3, David Millson4 and Peter Croft5:Substance misuse and psychiatric illness: prospective observational study using the general practice research database Journal of Epidimology 2005

    So in God I trust and will wait until all evidence has been presented

    Mickey Mouse

  5. Hi Mr Mouse

    Thing to remember is conclusions have already been drawn on the issue of cannabis and mental health – to the extent that laws have been changed on the basis of public concern whipped up by unsupported claims.

    The fundamental claim being made by the reefer madness campaign was that cannabis use would lead to am increase in psychosis. The one finding these claims simply cannot withstand is evidence on no such rise, let alone something of a fall.

    That said I will admit the closing para was perhaps a little too definite where I wrote:

    “what this shows is that increased use of cannabis has not lead to an increase in the rates of the illness” – I should of course have said “what this indicates is that increased use of cannabis has not lead to an increase in the rates of the illness” – it is, after all, only one study.

  6. halo, i’m Adam Hope, currently studying at Keele Uni, and i’m also the Keele Chapter Representative for the Students for Sensible Drug Policy UK. thanks for puttin up this study. i was aware of a study of this kind goin on at Keele, but have been unaware of the conclusions, which appears to be evident why i didn’t know. i am planning on carrying out my own research project next year, probably along the lines of analysing factors contributing to what makes people take drugs, a wide area i know. i will try to find the Keele paper and try to contact the researchers involved, to see what they concluded, and what they thought about having their research ignored by the Home Office. i am unsure if i will be able to acquire a copy that i can send you, as it sounds like their may be some restrictions on its distribution (which there shouldn’t be as this kind of research benefits everyone) but i will try my best.

    i hope everyone is well

    Adam 🙂

  7. Caffeine is a speed type substance, that causes psychosis, why is this variable ignored?

    For instance to get true data, on this you would have to use cannabis users that didn’t take the drug caffeine!

  8. The Keele study did what such studies always do, it tested an hypothesis. The claim was that if cannabis did indeed cause serious mental illness in healthy people then the rates of such illness would rise in step with increased cannabis use. That increase wasn’t seen hence the hypothesis wasn’t supported.

    The study does make reference to external factors – in this case factors which could have countered any increase caused by cannabis – and explains that is outside of the scope of the study.

    Of interest for me is the issue of army vets returning home from the varoius wars of that period, which might have been expected to have produced a rise in mental health problems themselves…

  9. Thats one side of the story, here is the other half.
    The webpage at the bottom is a newly available study avaialable online since July 29 2009 it looks at the statistics of cannabis users and mental illness/psychosis and, found that over a ten year period there has not been a significant increase in mental illness/pyschosis that would indicate that cannabis causes/increases mental illness/. Also to FRANK, and other goverment misinformation. You will only annoy people when they find out the truth and this will render your points whatever they be useless.

  10. | am sure that statistics could be found to support either side of the debate.Whether or not cannabis use has any health concerns attached can be put into perspective when we view cannabis along side thigs like alcohol, tobacco & caffein. I doubt very much wether anyone would dispute the fact that alcohol & tobacco are responsible for a huge number of deaths & illnesses annualy around the world. Why is there no great concern about the risks to our health when it comes to putting flouride in public water? A toxic industrial waste product that is the main ingredient of rat poison. If legalized, The benefits of hemp & cannabis production would far outweigh any disadvantages.
    The laws that govern cannabis serve only to protect profits not people.

  11. Present cannabis law is based on misinformation
    & government propoganda.More harm is being done by the law than cannabis.Alcohol is far more harmful.Also cannabis has many medical benefits.
    The present situation is a tragic farce.

  12. Would you let your OWN child smoke or eat cannabis? Would you encourage your OWN child and tell him/her there is nothing wrong with doing that?

    There is this tall, blue eyed, handsome man living in London who smokes it daily. He now works at a hospital and claims cannabis is not harmful. Yet, after smoking it for many years, whereas he used to display intelligence, now he cannot understand simple math equations and worse, misinterprets 80-90% of very simple sentences. He has other visible signs of physical, mental malfunction problems. No, do not think that, there is palpable evidence (proof) he was not always like that. It was only after the years of use that his brain became “addled”. He always keeps a good size stash in his flat, smokes daily and smells like pot, yet he wants to get married and have children! As a woman, would you have his child, that is, knowingly let YOUR child being created with such sperm and worse, be raised by such a man?

    If you do get pregnant from such a person, it is better to give the child up for adoption so that he can never have a hand in raising a child who already has problems just by being created by such defective sperm.

    I wonder if a study has been made of people’s IQ and other mental capacity and intelligence measurements at age 20 and again 20 years later, with half the test subjects using cannabis daily and the other half not. The people I have observed who used cannabis for many years display paranoia, some sociopathic symptoms (no consideration of others’ feelings and others), obvious narcissistic actions and reactions to everyday functions, some with daily use also display some schizophrenic symptoms.

  13. Wondering

    I’ve combined your four comments into one, please don’t post multiple comments.

    To your first point – and if you’d taken the time to read around this site and it’s blog you would know that one of the prime aims of UKCIA is to see a properly regualted trade in cannabis, which would include age limits for purchase. So no, children shouldn’t using cannabis. Under prohibition, there is nothing to stop them however.

    As to your othe points I think they are quite distasteful really and there have always been people with mental heatlh problems as you describe, long before cannabis became popular.

    And yes, there have been several studies of cannabis use and intelligence, check out the research section of UKCIA

    It’s strange isn’t it how some people seem to know lots of cannabis users who became mentally harmed in some way, whereas most of us who have been close t the culture for some time never met any.

  14. Well, in all fairness, have we all forgotten that the average drug indulger almost certainly has a predisposition to those habits by way perhaps of genetic carry-over by the practices of their biological parents present or past or perhaps even during pregnancy. Couldn’t it also be plainly by way of what they were exposed to by their parents and/or guardians (many times from their early years) who most certainly knew that it was not right to expose the kids to their practices let alone the smoke itself and justify their arrogances with the line “cannabis was harmless” even if most, (the honest ones at least) are quite prepared to agree that they have at least once or twice witnessed episodes of unexplainable behaviors by some of their mates (co-users)to which one could only refer as some level of over indulgence or the result of. Therefore, when the inner-self puts on an unexplainable song and dance (often unrecallable) by the doer, one would find it hard to say anything other then “he/she had a brain snap. Someone please tell me that, that is anything other than a mental episode. Long before this episodes come around, back in the early days of the practice, another episode (which most choose to ignore) had already taken it’s place in the users mind and that is the onset of laziness, sometimes extreme laziness, which is usually the reason why they couldn’t be bothered in the first place to hide the practices from their families and/or friends and hence their offsprings copled to whatever justifications they could muster, reguardless. Someone tell me please, that, that almost uncontrollable (verdging on irresponsible) onset of laziness is not a form of mental imbalance particularlly when one knows that the responsibilities they are ignoring or have choosen to ignored were simply theirs alone. Have we forgotten absent-mindedness, forgetfulness, irritability, short-temperedness, irrationality, sleeplessness, diminishing selfrespect and on and on and on. Might I add too, especially for those of us who have the habit of selective memory lose, that most indulgers usually dabble in most other so called “mild-happy” drugs that come about, or wouldn’t think twice before having a try. Many times, this is only the begining and it doesn’t usually end there. This goes without saying that most users also use either coffee, tea, tobacco, alcohol, tranquillizers, sleeping pills, painkillers and others substances or all of the above. Wasn’t the revered saying “Too much of anything is poisonous”. I’m certain that rule applies in this instance without a doubt. I dare say that there is a lack of genuine honesty on both sides of the debate, particularly when it comes to bearing in mind the extent to which the habit of drug taking very commonly developes as well as the commonality of the cocktails used that become the norm and the varied changes that most users experience in themselves though many don’t realize the changes and alot may not necessarily admit to. I’ve been there and done that and trust me when I say most of those mates I had who were not able to let go suffered and many are still suffering the result of varying attitudes and reasoning etc, etc, etc. Many now live alone.

  15. JMMB

    You wrote

    Well, in all fairness, have we all forgotten that the average drug indulger almost certainly has a predisposition to those habits by way perhaps of genetic carry-over by the practices of their biological parents present or past or perhaps even during pregnancy

    Where did you get that from? Total fabrication I’m afraid.

    You should be aware that the vast, vast majority of cannabis users suffer no ill-effects at all from their cannabis use.

    Of course you can overdo almost anything.

  16. When I initially commented I clicked the -Notify me when new feedback are added- checkbox and now each time a remark is added I get 4 emails with the same comment. Is there any means you can take away me from that service? Thanks!

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