Study shows cannabis users have a markedly higher chance of developing psychoses

nesf

http://www.newscientist.com/article.ns?id=dn12363&feedId=online-news_rss20

41% more likely to develop psychoses in later life apparently.

hoolio

I cant seem to get the latest issue of lancet online yet,, and i always take summaries like the one linked with a rather large pinch of salt. Although to be fair newscientist.com is a better source then the herald am bollocks that warrant my salting policy.

Judgement reserved till i poke me way through the article, but i guess assuming the article is accurate and the study is sound i wouldnt be too shocked. Add it to the long list of things that many people do today that might come back to bite us on the arse. Might add some fuel to the whole legisation debate i guess. Although to be honest i doubt it'll be so clear cut as a simple 41% rise when you delve into it deeper. Time will tell

Edit: forgot to point out that seeing as it isnt a discrete individual study and is more an analysis of other studies trying to draw out common threads, leads to more problems, but once again, i havnt read it yet, so not much non supposition to say.

John

It's up now (Moore et al [2007] Cannabis use and risk of psychotic or affective mental health
outcomes: a systematic review. The Lancet, 370:319-328). Here's the abstract:

Moore et al, 2007 wrote:

Background Whether cannabis can cause psychotic or aff ective symptoms that persist beyond transient intoxication is
unclear. We systematically reviewed the evidence pertaining to cannabis use and occurrence of psychotic or affective mental health outcomes.

Methods We searched Medline, Embase, CINAHL, PsycINFO, ISI Web of Knowledge, ISI Proceedings, ZETOC, BIOSIS, LILACS, and MEDCARIB from their inception to September, 2006, searched reference lists of studies selected for inclusion, and contacted experts. Studies were included if longitudinal and population based. 35 studies from 4804 references were included. Data extraction and quality assessment were done independently and in duplicate.

Findings There was an increased risk of any psychotic outcome in individuals who had ever used cannabis (pooled adjusted odds ratio=1·41, 95% CI 1·20–1·65). Findings were consistent with a dose-response eff ect, with greater risk in people who used cannabis most frequently (2·09, 1·54–2·84). Results of analyses restricted to studies of more clinically relevant psychotic disorders were similar. Depression, suicidal thoughts, and anxiety outcomes were examined separately. Findings for these outcomes were less consistent, and fewer attempts were made to address non-causal explanations, than for psychosis. A substantial confounding eff ect was present for both psychotic and aff ective outcomes.

Interpretation The evidence is consistent with the view that cannabis increases risk of psychotic outcomes independently of confounding and transient intoxication eff ects, although evidence for aff ective outcomes is less strong. The uncertainty about whether cannabis causes psychosis is unlikely to be resolved by further longitudinal studies such as those reviewed here. However, we conclude that there is now suffi cient evidence to warn young people that using cannabis could increase their risk of developing a psychotic illness later in life.

I haven't read the article yet but it's just a review/meta-analysis, so basically a reiteration of what has already been said and a way of spotting any patterns emerging from various different studies that have previously been published. I'll give some thoughts tomorrow when I've read the article and had some sleep.

tallaght01

I've had only the briefest look at the study today, as I'm just out of work.

It seems to me that we already know that there's an association between drug use and psychotic illnesses. However, the problem was knowing whether it was chicken or egg. ie are those who are in the early stages of psychosis more likely to turn to cannabis anyway?

This question still hasn't been answered.

I guess the only way to answer it for sure is to randomise people to using cannabis or a placebo, and that's never going to happen.

Having said that, the finding that there's a dose-relationship certainly helps the theory that cannabis leads to mental illness, as does the fact that cannabis users develope psychoses at a younger age.

wheresthebeef

seems like a useful retort to the usual tripe that cannabis users come out with: "it's a safe drug". There still remains a lot of false information in the public domain about using cannabis.

DrIndy

Its interesting alright - remember that there are personality traits there - schizophreniform, depressive, anxious, mania, borderline etc....

These are tendencies for people to think along certain pathways and they are an innate psychological vulnerability to certain psychiatric disorders. Depressive trait people are knocked low by things most people would brush off - but still have a normal social and psychological function and are not depressed - however, if they encounter a major life trauma - they have a much greater likelihood of being unable to cope and as a result develop clinical depression.

Likewise, people who have a schizophreniform trait are "odd" and think along a different track from people and interpret events differently than others would - they may be suspicious of others and think people may be plotting against them - but are not affected in their personal interaction. these people would therefore be at a greater risk of developing a full blown psychoses under the right circumstances.

Cannabis can be the trigger. A hypothesis therefore is long term cannabis use poisons all people's minds - but is only manifested in those who have a trait for schizophrenia.

pharmacien

41% more likely to develop psychoses in later life.

This seems to be the line taken by the majority of the media. Shame they didn't even bother to read the first line of the paper (never mind the rest of it!!!): "Whether cannabis can cause psychotic or affective symptoms that persist beyond transient intoxication is unclear."

A good critical appraisal of the paper was published in yesterday's Guardian in the Bad Science column (which is always excellent in it's coverage!). He compares the paper to the coverage given by the Mail (and Irish Mail)

"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."