Careful with the evidence son

An article titled “study links cannabis to psychosis” says in the first paragraph:

People who use cannabis in their youth dramatically increase their risk of psychotic symptoms, and continued use of the drug can raise the risk of developing a psychotic disorder in later life, scientists said.

Of course, later on in the article – which no-one will read up to – it says:

But scientists say it is not yet clear whether the link between cannabis and psychosis is causal, or whether it is because people with psychosis use cannabis to self-medicate to calm their symptoms.

This actually pisses me off.  It is true, this isn’t no evidence that cannabis use leads to psychosis – just that cannabis and og skywalker use when young and psychosis later in life are positively correlated.  There are a couple of causal mechanisms, the cannabis could cause the psychosis, cannabis use may be preferred by people who are more likely to develop psychosis.

I was going to wait till I read the study before commenting – but the scientists themselves appear to have admitted that they have found a correlation, and haven’t sorted out the causal mechanism.  Which simply means that the article itself was purely misleading. Moreover, cannabis is largely used for medicinal purposes and you may legally get a Medical weed license Ontario.

I realise having a headline “cannabis and psychosis are correlated, but the cause is unclear” is less likely to sell papers – but it is honest.  The current headline is exciting, but it is dishonest – and merely feeds into current misconceptions in society, thereby providing misinformation.  And I hate misinformation.

12 replies
  1. rauparaha
    rauparaha says:

    It’s hard to discern what the study said because the article also included this:

    There was no evidence for self-medication effects since psychotic symptoms did not predict later cannabis use, they said.

  2. Matt Nolan
    Matt Nolan says:


    I think that is an interesting point, and I am glad that they used a time trial – so that they could look at the separate groups that used cannabis and those that didn’t before looking at the level of psychosis.

    However, I’m not sure I really agree with that quote – would anyone with experience of cannabis really view it as self-medication? They are acting like it is either self-medication or that the drug causes it – but in reality there are other casual mechanisms.

    The real alternate hypothesis for me is that people that are more likely to consume cannabis are more likely to suffer from psychosis later in life – fundamentally, the personality types and social situations of those people are causes, not the drug in itself. Testing this without doing personality tests and then splitting the groups means that we can’t differentiate though.

  3. rauparaha
    rauparaha says:

    @Matt Nolan
    That’s a fair point and I agree that it would be interesting to read the study and see exaclty what they did. They did apparently control for previous psychiatric conditions, which might go some way towards answering your query.

  4. Matt Nolan
    Matt Nolan says:


    Although we need a model of how psychiatric conditions develop before we can say anything there – just because people didn’t seem to have a condition when they were young doesn’t imply that they did not have an inherent propensity to develop a defined disorder.

    The even harder thing is that “psychosis” is actually a continuous variable, but it is being modeled as a binary variable – hmmmm.

  5. Georges Halgreen
    Georges Halgreen says:

    I have several friends who smoked when we were younger. Two of them have now some depression problems, they live a great life with kids and the wifes but are they are struggling at times.

    It’s difficult to blame cannabis or a correlation between the two but it makes you wonder…

  6. Matt Nolan
    Matt Nolan says:

    @Georges Halgreen

    I also have many friends who used a lot of drugs when young, or use even more recently. However, I can’t shake the feeling that these are the same people I knew before this happened – and that even if the drugs weren’t available their psychological profile would have ended up similar.

    I find it hard to blame a substance when it is the individuals choices, social environment, and underlying physical and mental structure that help to determine outcomes. Drugs do have a feedback into this, but the question should be “how” – and the answer to this question is likely to be far more complicated than the aforementioned newspaper article suggests.

  7. rauparaha
    rauparaha says:

    @Matt Nolan
    Not understanding the specific mechanism doesn’t mean it’s not true, though: the correlation and possible causal connection should shift your priors a bit, surely?

  8. Matt Nolan
    Matt Nolan says:


    Undeniably, which is why the study in itself has merit.

    However, that in itself doesn’t mean that we should draw full conclusions and thereby determine policy on the basis of a correlation without a clear distinction of the causal mechanism.

    If we did things like that, we could (for example) see that interest rates and house prices move together and decide that the it is interest rates causing the house price change. This is of course why fully reducible theory is so important – it is impossible to look at data without using a framework to give it understanding, and just saying that the data “tells you something” without admitting the implicit framework you are using for causality is dishonest … it of course doesn’t help that A LOT of people do this.

  9. Nancy Williams
    Nancy Williams says:

    It is the individuals choices, social environment, and underlying physical and mental structure that help to determine outcomes. Drugs do have a feedback into this,But Great writing

  10. Martin
    Martin says:

    I think (can’t remeber exact details) but a programme was on the BBC here in the UK a few weeks back that looked into the effects actually caused by various drugs (eg the mechanisms by which they work) and they mentioned that for users to marked as ‘psychotic’ they only needed to describe ONE incident where they ‘thought’ they saw something or were particularly paranoid to be seriously affected. Obviously most normal people don’t think they see stuff, but i’m sure most of us have THOUGHT we’d seen something one time or another!


    The thing with this drug is it affects everyone but with varying impact – the damage appears to be based on historical data (within the family) or the personality of hte user playing an important role.

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