Preventing cannabis use will not have much effect on rates of mental illness – study.

Just how many people would you need to prevent getting stoned to have any hope of reducing the rates of mental illness if the theory of a causal role for cannabis were true? The answer, it seems, is rather a lot.

A strange bit of research was published this week by researchers at Bristol, London, Cardiff and Cambridge Universities. Amongst the names involved was one Stanley Zammit who got a metion a couple of weeks back for demonstrating (against his expectations) that cannabis use during pregnancy doesn’t seem to be related to psychosis later in the offsprings life.

Dr Zammit, as this has blog already observed, has been involved in the cannabis and mental health debate for some time and seems to be of the opinion that there is a causal link between cannabis use and the development of severe mental illness. Indeed, he published a study a few years ago which was widely reported as claiming to have found that those who had used cannabis were 41 per cent likelier to experience an episode of this kind than people who never smoked.

This latest bit of research seems designed to quantify what that means in terms of public health policy: How many people would you have to prevent using cannabis in order to reduce the incidence of severe mental illness?

In order to do this, they invented a new scientific term, the “NNP” – defined as “The number of cannabis users needed needed to prevent  one case of severe mental illness”. Now in all honesty when terms like NNP start getting used the UKCIA “Cod science” alarm bell starts ringing and it’s fair to say it hasn’t really stopped, despite us quite liking the results.

The study was called “If cannabis caused schizophrenia—how many cannabis users may need to be prevented in order to prevent one case of schizophrenia? England and Wales calculations” and the abstract – for what it’s worth – can be seen here although if you get a cookie error message you may have to go to the front page of Wiley Interscience and do a search for “cannabis schizophrenia”to actually get to the page.

The first problem with this effort is in the title – “If cannabis caused  schizophrenia” implies an assumption which begs a lot of questions in and of itself, not least of which is what do they mean by cannabis? Do they mean all cannabis or only the dreaded “killer skunk” of Daily Mail fame? Sadly the abstract doesn’t give any clues as to what they mean by “If cannabis caused schizophrenia” and the article isn’t on open access so we can’t get to it. But presumably they laid out their theory as to what the nature of the  causal role  would be.

So anyway, the object of the study was to determine the NNP of cannabis (don’t you just love science jargon) – just how many people would you need to keep away from cannabis to reduce the incidence of schizophrenia by one. The results make uneasy reading for those who campaigned so strongly on the cannabis and mental illness ticket and seriously throw into question the government’s logic reclassifying cannabis because of the concerns expressed in the Daily Mail about this issue.

Results  In men the annual mean NNP for heavy cannabis and schizophrenia ranged from 2800 [90% confidence interval (CI) 2018–4530] in those aged 20–24 years to 4700 (90% CI 3114–8416) in those aged 35–39. In women, mean NNP for heavy cannabis use and schizophrenia ranged from 5470 (90% CI 3640–9839) in those aged 25–29 to 10,870 (90% CI 6786–22 732) in 35–39-year-olds.

Equivalent mean NNP for heavy cannabis use and psychosis were lower, from 1360 (90% CI 1007–2124) in men aged 20–24 and 2480 (90% CI 1408–3518) in women aged 16–19. The mean and median number of light cannabis users that would need to be prevented in order to prevent one case of schizophrenia or psychosis per year are four to five times greater than among heavy users.

So in other words for adults over 20 or so you would have to prevent around 3,000 heavy cannabis users, or 150,000 light users. If you use a wider definition of “psychosis” rather than “schizophrenia” the numbers are lower and they are lower for younger aged people. Even so these numbers hardly demonstrate a strong causal relationship and in all honestly putting a lot of effort into prevention of cananbis use isn’t going to have that much effect on the rates of serous mental illness. All this, of course, working from the assumption that there is indeed a causal  role at work here at all.

The conclusions drawn in the study state:

The number of young people who need to be exposed to an intervention to generate NNP and prevent one case of schizophrenia will be even larger. The public health importance of preventing cannabis to reduce schizophrenia or psychosis remains uncertain. More attention should be given to testing the hypothesis that cannabis is related causally to psychotic outcomes, and to considering what strategies will be the most effective in reducing heavy cannabis use among young people.

An English translation of this might read something like:

The number of young people who would need to be  exposed to anti cannabis use programs would be even larger than the figures quoted here, it would not seem to be a sensible way to spend the money. There is little evidence that cannabis does cause schizophrenia and policies designed to reduce the use of cannabis are not likely to have much effect on the rates of the illness.  More effort needs to be put into trying to prove a causal link between cannabis use and schizophrenia because we haven’t managed to prove it yet. But it’s a good idea to reduce heavy use amongst young people anyway.

If that is a fair translation of the conclusions of this rather odd study then no-one could really disagree with the last sentence. But it is yet another study which has failed to support the cannabis and mental illness scare of a few years ago to add to the ever growing list.

A question not asked and apparently not considered in all of this is what would the effect on the rates of mental illness be if cannabis users were to be persuaded not to use tobacco? Did they even consider the role of tobacco in mental illness? There’s no way to tell from the abstract but the answer is probably “no”, after all if the elephant in the room is really that big, it simply can’t be considered.


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.

11 thoughts on “Preventing cannabis use will not have much effect on rates of mental illness – study.

  1. This was also looked at in the ACMD report which stated cannabis should remain class C. This is not cod science, it is epidemiology. Similar data modeeling applies to statin drugs and vaccines in a population. I agree with your contention that the debate is skewed but you should’nt cherry pick findings because you like the results.

  2. What produced the “cod science” comment was the invention of the “NNP” term, along with the title of the study. Check out previous blog entries – and the site gnerally – and I think you’ll see UKCIA tried not to cherry pick favourable results overall.

    So if cannabis does not have a causal role, would the results still be valid? Your comments seem to imply they would be, if so, why would they be valid?

  3. The term NNP makes perfect sense to me, number needed to prevent. It is a very common term and statistic in clinical trial resesarch, usually called number needed to treat or NNT. It is a measure of how potent a treatment is in terms of how many people you need to treat before expecting to see one person with positive benefit. It is also a very simple way to express more fancy (and less interpretable) statistical measures. All the authors did is call it NNP since they are looking at prevention.

  4. Stephan is correct – NNP is an entirely rational name what they’re doing. NNT is a very common and extremely useful statistical measure and there’s nothing ‘cod science’ about it.

  5. Which probably explains why I couldn’t find the term NNP listed anywhere and indeed have never heard of it. The term was introduced as a new term in the abstract as being one defined within this study.

    Indeed, has it ever been used before, even as a concept?

  6. Cannabis may or may not cause schizophrenia, but it certainly causes psychosis. People aren’t always comparing like with like – much of the cannabis for sale now is of a strength that just wasn’t available as little as 20 years ago. What do we do – tolerate lives being ruined so that people can assert their spurious right to commit an illegal act?

  7. What we do is control and regulate the product Frugal. If there is any truth in the claim you make of a massive increase in strength in recent years, that’s happened under your regime of illegality, it apparently – according to people like you – never happened in the thousands of years before, did it?

  8. right on ukcia! gotta love this site, all the law reform ammunition you could ever need!

  9. As far as I’m aware skunk has only been in the UK since the early 1990s; in other places they’re content with the plants as they were. For example, I’ve seen people coming from Africa to Cambridge to work or study, having only experienced “bush-weed”; they never had a chance when explosed to the selectively-bred material over here, which no amount of control or regulation will exclude from the marketplace as long as there’s money in it.

  10. so basically frugal what your saying is you don’t mind children smoking weed? and you don’t mind that weed being illegal is causing the general populus of tokers alot more health defects asscioated with adulterated weed and lack of proper education than it would if it was legal and our weed was clean and un-sprayed/un-adulterated?

    weed was only prohibited in the 1930’s in the US because harry anslinger and a few other high up select individuals in goverment were scared that hemp fiber production and hemp paper production would threatern there own nylon oil based fiber production companys and there own printing companys. this is what caused the huge wave of anti-cannabis propaganda in the 1930’s known as reefer madness (named after the classic propaganda film of the same name). So basically there was a reason for its prohibition back then (be it a very corrupt one indeed) but now its just pointless to leave it prohibited and it just shows the obvious fact that goverment doesn’t care either way about the enjoyment of our citizens. Ever since the inital outlaw in the UK and US they have been trying to find scientific proof for the adverse effects on human health but even compared to alcohol a legal drug the effects are not really compareable. All you need to do is look at the death statistics for direct use of alcohol and compare them to deaths caused by direct use of cannabis and it just shows that the fact its still illegal is completely illogical.

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