Last week in this blog I made an appeal for people to send in a copy of the Keele study “Assessing the impact of cannabis use on trends in diagnosed schizophrenia in the United Kingdom from 1996 to 2005” by Martin Frisher Ilana Crome, Orsolina Martino and Peter Croft and it was heartening that so many people obliged (thank you all), which allows a second look at the study and an explanation of the mystery presented in last weeks blog.
First of all, lets clear up the mystery. In last weeks blog, we reported several sources as reporting that the study came to the conclusion that:
“[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.”
This specific quote came from the American NORML blog, but was also reported elsewhere
The mystery was that this very interesting conclusion – which is central to the whole point of the study – was not mentioned on the abstract (summary) of the study, which is the only part of the study available to the public and perhaps most importantly, the media. In fact the abstract stated as a conclusion only that:
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.
We can now confirm that the abstract does indeed fail to carry the full conclusions of the study, which is highly suspect in all honesty
The study actually concludes:
4.5. Conclusions
The most parsimonious explanation of the results reported here are that the schizophrenia/psychoses data presented here are valid and the causal models linking cannabis with schizophrenia/psychoses are not supported by this study.
As that was the prime objective of the study, it’s hard to understand why it was not included in the abstract.
In the interests of total openness, the rest of the conclusion states:
A number of alternative explanations have been considered and while they cannot be wholly discounted, they do not appear to be plausible. There are also other
possibilities, for example causes of schizophrenia/psychoses may have declined thereby masking any causal affect of cannabis use on the prevalence of schizophrenia/psychoses. However, it is beyond the scope of this study to examine this hypothesis and we are therefore left with the most parsimonious explanation, namely that the underlying causes of schizophrenia/psychoses remained stable/declined over the study period.
The important point is the Keele study failed to support the claims of a causal role of cannabis in the development of psychosis, that really is the headline conclusion and is important because the study was done as a part of the Home Office review in to the role of cannabis and severe mental illness.
The quote featured on the NORML blog actually comes from the discussion section of the paper, the full quote is this:
4.2. Links with trends in cannabis use
In terms of the model set out in the Introduction, the 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 the incidence of psychotic disorders based on the 3 assumptions described in the Introduction. This concurs with other reports indicating that increases in population cannabis use have not been followed by increases in psychotic incidence (Macleod et al., 2006; Arsenault et al., 2004; Rey and Tennant, 2002).
However, it is not in line with findings of a rise in first admission rates for psychotic disorders among young people in Zurich following increases in cannabis availability and consumption (Ajdacic-Gross et al., 2007). One factor involved in this discrepancy may be the potency of the cannabis consumed, which varies considerably within Europe (EMCDDA, 2008). In addition, a Netherlands study found that high-potency cannabis obtained from ‘coffee shops’ led to higher levels of tetrahydrocannabinol (THC) in the blood, with young males aged 18–45 at particular risk for excessive consumption (Mensinga et al., 2006).
The 3 assumptions described in the Introduction are:
a) an elevated risk of developing schizophrenia/psychosis among cannabis users,
b) a substantial rise in cannabis use in the UK from the mid-1970s onwards and
c) an assumed elevated risk of 20 years,
this model would predict a corresponding increase in schizophrenia/psychosis during our study period.
At this point is would be good to look at the studies mentioned here (full references at the foot of this blog), in particular the Zurich study which claims to have found just such an increase in psychosis. The Zurich study is entitled “Changing incidence of psychotic disorders among the young in Zurich” and is also published by Schizophrenia Research. Google finds the following abstract:
There is controversy over whether the incidence rates of schizophrenia and psychotic disorders have changed in recent decades. To detect deviations from trends in incidence, we analysed admission data of patients with an ICD-8/9/10 diagnosis of psychotic disorders in the Canton Zurich / Switzerland, for the period 1977–2005. The data was derived from the central psychiatric register of the Canton Zurich. Ex-post forecasting with ARIMA (Autoregressive Integrated Moving Average) models was used to assess departures from existing trends. In addition, age-period-cohort analysis was applied to determine hidden birth cohort effects. First admission rates of patients with psychotic disorders were constant in men and showed a downward trend in women. However, the rates in the youngest age groups showed a strong increase in the second half of the 1990’s. The trend reversal among the youngest age groups coincides with the increased use of cannabis among young Swiss in the 1990’s.
Now given the slightly misleading nature of the Keele abstract, it would be nice to see the Zurich study, but it costs $31.50 just to read, so like the Keele study it’s hardly in the public domain.That’s a pity and it’s odd that such science is so restricted, especially when these results are used to justify criminal sanctions.
A couple of points from the Keele study which also need raising.
No-one has ever suggested cannabis leads to immediate mental health problems. Therefore the Keele study which covered the period 1996 – 2005 in fact looked at the results of people who began using cannabis between something like 1994 – 2003 and wouldn’t (on the whole) have included people who started after about 2003. This is interesting for a couple of reasons:
1: It covers the period before reclassification to class C and the claimed drop in use of recent years. This was a period of great increase in the use of cannabis – and the other “rave” drugs by young people.
2: It quite neatly covers the arrival of so-called “skunk”, which was rare in 1994 and pretty well complete by 2000. That there was no change in that time frame would seem to undermine the claims of a greater danger caused by high THC levels with lower levels of CBD. If these claims were true, we would have seen an upturn in the rates of psychotic illness for that reason alone, never mind the general increase in use.
By way of explanation as to the difference found between this study and the Zurich study, the keele study states:
However, it is important to take into account sociocultural differences in cannabis availability and consumption; for this reason, it is more appropriate to focus on trends in cannabis use in the UK when comparing these to schizophrenia/psychosis trends in UK general practice.
and
One factor involved in this discrepancy may be the potency of the cannabis consumed, which varies considerably within Europe (EMCDDA, 2008).
In Switzerland the whole illegality of cannabis declined markedly during the 1990’s. The implication being made – and clearly made by the abstract of the Zurich study – is that greater availability has lead to more young people being harmed by exposure to the cannabis trade and a higher potency product that open trade produced. However, since the Keele study was done, we have now learned (officially) that the same thing has happened here, with high THC “skunk” replacing the traditional imported hash. We also know that cannabis use by minors in the UK is unacceptably high, having increased greatly through that time frame.
An alternative explanation could be that the greater openness in Switzerland due to the removal of legal sanctions means people are more willing to come forward and describe issues to their doctor, whereas here the law acts as a a deterrent to reporting such health issues.
In conclusion this report should be in the public domain and the abstract should be rewritten to more accurately reflect the findings of the study. On a general note it’s somewhat depressing to see scientific research blocked from open view by issues of copyright ownership. This certainly should not be happening with research carried out with public money on behalf of our government, as is the case with the Keele study.
——————————-
Studies into rates of psychotic illness cited:
Macleod, J., Davey Smith, G., Hickman, M., 2006. Does cannabis cause schizophrenia? Lancet 367, 1055,
Arsenault, L., Cannon, M., Witton, J., Murray, R.M., 2004. Causal association between cannabis and psychosis: examination of the evidence. Br. J. Psychiatry 184, 110–117,
Rey, J.M., Tennant, C.C., 2002. Cannabis and mental health. BMJ 325,1183–1184.
Ajdacic-Gross, V., Lauber, C., Warnke, I., Haker, H., Murray, R.M., Rössler, W., 2007. Changing incidence of psychotic disorders among the young in Zurich. Schizophr. Res. 95, 9–18.
all of your blogs are amazing and essecental for spreading the TRUTH!
for so long we have been spoon fed propaganda and lies about cannabis.
prohibition has to end cannabis clearly isn’t as bad as the other widely used recreational drugs. because of prohibition we have all had to put up with bullshit now for years. i cannot smoke anymore [not even cigs] because of sprayed weed. i have lung pain everyday now because of the contamination problem that could be stopped in an instant by legalisation. thank you goverment for taking another peice of fun out of my life. looks like now i have to spend my days drinking brain destroying poison and becoming instinctivly voilent for no reason.
as for mental illness i’ve never really experianced any mental problems due to cannabis use or known anyone that has.
“tingtongstonerbong” – Sorry to hear about your state of health – There must be many more like you or at least there will be in the future. I was lucky because I gave up buying cannabis (a long time ago) before the contamination got too bad but the stuff I see friends put up with some times makes my skin crawl. Then what will they say – it was harm caused by cannabis. At least you are honest about where the blame lies for all this.
Anyway it need not end your fun! check out the main page of this site for other ways to enjoy your favourite herb (Follow the link – “you don’t have to smoke”). It certainly beats using the poisonous liquid that you allude to in your comment !
I’ve always been a bit skeptical about the cannabis mental illness thing. I smoked cannabis, was diagnosed with schizophrenia and told it was due to cannabis use. I’m fine now and still smoke cannabis. The link between the two has never made sense to me.
Highlyme, that is because there is nothing to prove the link. Schizophrenia is by far usually down to social problems in the earlier stages of life. Double bind situations and not giving the proper attention to your children is one of the key causes. Its a confused mindstate that can grown with time or lay dormant until something triggers it off.
But through all my time dealing with people on a 1-to-1 basis, i’ve found by far, those who do suffer from mental health issues often gravitate towards cannabis as it somehow manages to regulate and help thier condition.
However this is not true of all and can sometimes be put down to the drugs they switch to or even smoking contaminated cannabis or badly grown and chemically filled cannabis/soapbar. It can often be the additives in these that trigger bad bouts of paranoia, the sweats, bad throats and such. Often irritability as your mind needs help flushing the problems and they are only made larger by cannabis that does not work or contaminants that make it worse.
I just wish the government would hold a small panel of members of the public and members of transform to have a proper debate and see where the actual ball lays.
I dont want to see the youth of today damaged beyond repair by increased risk of coming into contact with contaminated substances that may well in the long run result in increases in mental issues. However as the facts have stated all along, clean cannabis used in moderation by the majority is NOT an issue and as such should be put up there with alcohol and tobacco of which cause much greater harm and damage to society.
Its time to flush the uk of criminal markets in susbtances that we consume. We have the right to experiment with our bodys and minds as long as we do not harm another being, human/animal or otherwise. I for one would support a local coffee shop which sells cannabis of a high clean standard. I would rather not deal with scum who sell to those underage and ruin thier chances of a future.
Like another great individual, “I too have a dream!”
it’s about time there was a SERIOUS study into cannabis. Not a biased one. Simply can’t understand why cannabis is forbidden and alcohol is not while the latter is far more dangerous to consumer and his surrounding.