Young cannabis smokers run risk of lower IQ, report claims


Prof Terrie Moffitt, researcher: "Those who started using cannabis regularly when they were in secondary school had lost, on average, about eight IQ points"

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Young people who smoke cannabis for years run the risk of a significant and irreversible reduction in their IQ, research suggests.

The findings come from a study of around 1,000 people in New Zealand.

An international team found those who started using cannabis below the age of 18 - while their brains were still developing - suffered a drop in IQ.

A UK expert said the research might explain why people who use the drug often seem to under-achieve.

For more than 20 years researchers have followed the lives of a group of people from Dunedin in New Zealand.

They assessed them as children - before any of them had started using cannabis - and then re-interviewed them repeatedly, up to the age of 38.

Having taken into account other factors such as alcohol or tobacco dependency or other drug use, as well the number of years spent in education, they found that those who persistently used cannabis - smoking it at least four times a week year after year through their teens, 20s and, in some cases, their 30s - suffered a decline in their IQ.

The more that people smoked, the greater the loss in IQ.

Start Quote

It is such a special study that I'm fairly confident that cannabis is safe for over-18 brains, but risky for under-18 brains”

End Quote Professor Terrie Moffitt Institute of Psychiatry, King's College London

The effect was only noticed in those who started smoking cannabis as adolescents.

Researchers found that individuals who started using cannabis in adolescence and then carried on using it for years showed an average eight-point IQ decline.

Stopping or reducing cannabis use failed to fully restore the lost IQ.

The researchers, writing in the US journal Proceedings of the National Academy of Sciences, found that: "Persistent cannabis use over 20 years was associated with neuropsychological decline, and greater decline was evident for more persistent users."

"Collectively, these findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects."

One member of the team, Prof Terrie Moffitt of King's College London's Institute of Psychiatry, said this study could have a significant impact on our understanding of the dangers posed by cannabis use.

"This work took an amazing scientific effort. We followed almost 1,000 participants, we tested their mental abilities as kids before they ever tried cannabis, and we tested them again 25 years later after some participants became chronic users.

Start Quote

There are a lot of clinical and educational anecdotal reports that cannabis users tend to be less successful in their educational achievement, marriages and occupations”

End Quote Professor Robin Murray Instuitute of Psychiatry, King's College London

"Participants were frank about their substance abuse habits because they trust our confidentiality guarantee, and 96% of the original participants stuck with the study from 1972 to today.

"It is such a special study that I'm fairly confident that cannabis is safe for over-18 brains, but risky for under-18 brains."

Robin Murray, professor of psychiatric research, also at the King's College London Institute of Psychiatry but not involved in the study, said this was an impressive piece of research.

"The Dunedin sample is probably the most intensively studied cohort in the world and therefore the data are very good.

"Although one should never be convinced by a single study, I take the findings very seriously.

"There are a lot of clinical and educational anecdotal reports that cannabis users tend to be less successful in their educational achievement, marriages and occupations.

"It is of course part of folk-lore among young people that some heavy users of cannabis - my daughter calls them stoners - seem to gradually lose their abilities and end up achieving much less than one would have anticipated. This study provides one explanation as to why this might be the case.

"I suspect that the findings are true. If and when they are replicated then it will be very important and public education campaigns should be initiated to let people know the risks."

Prof Val Curran, from the British Association for Psychopharmacology and University College London, said: "What it shows is if you are a really heavy stoner there are going to be consequences, which I think most people would accept.

"This is not occasional or recreation use."

She also cautioned that there may be another explanation, such as depression, which could result in lower IQ and cannabis use.

Marjorie Wallace, chief executive of the mental health charity SANE, said: "In a significant minority of people who are vulnerable the drug can act as a trigger to illnesses like schizophrenia which may last a lifetime."

Illicit drug use by young people has been decreasing since the mid 1990s, but the rate of decline in cannabis use throughout most of the last decade has been slow, official statistics show.


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  • rate this

    Comment number 34.

    The "scientists" are really statisticians and churning 3rd hand anecdotes to justify their 'findings' Nice work if you can get it. Hardly science though.

    More a quiet news day methinks.

  • rate this

    Comment number 33.

    For goodness sake Beeb... lets call a spade a spade and a shovel hard work huh!

    It's not what you smoke, it's not what you eat but it's is certainly how you do it.... smoke like a chimney eat like a pig.

    So you are calling one or two of our pop superstars three sheets in wind.... a darn sight more entertaining than 'many' of your programs.

  • rate this

    Comment number 32.

    Firstly; If you are going to post on this site at least make it constructive, analyzing the evidence that they provide and not by your own experiences.
    This whole article reeks of bad science to me, or at least a bad interpretation of the original study, phrases like 'The more that people smoked, the greater the loss in IQ' suggest this. The original study is also not attached, nor available

  • rate this

    Comment number 31.

    Yeah, in todays society it's definately the questionable evidence against an already illegal drug that we should be highlighting.

    Not the small warzones that city centers become on weekends due to legal ones.

    If I were a government I know which I'd want to address for the safety of those not partaking in a vice.

  • rate this

    Comment number 30.

    Maybe we should ask why so many young people feel they need to escape reality into drink and drugs? Could it be that life at the bottom in the UK is so dreadful that this is a better alternative than dealing with it?

    Coming from working class roots I know that life in the UK for poor kids was a hopeless living hell, contrary to what the pampered toffs at the Daily Mail will tell you.

  • rate this

    Comment number 29.

    4. Anthony

    "Biggest load of tosh, I've ever read, the scaremongers are out again!"

    Really? Perhaps it is more sensible to actually look at the evidence, and by all means draw your own conclusions and ignore or follow advice. But as stated in the article the data is very good. The authors of the study are not scaremongering, they are simply reporting their findings. "Tosh" it isn't.

  • rate this

    Comment number 28.

    I think it's fair to say cannabis certainly isn't good for you, much like alcohol, cigarets and all the other social poisons we fill ourselves with.

    I do however think the government are missing a missive trick here in legalising it. The tax revenues would probably surpass those imposed on tobacco, and it would help drive filthy drug dealers out of business.

  • rate this

    Comment number 27.

    I can't say that I approve of the subtle innuendo that have having a low IQ is something inherently bad. We can't all be Einsteins. Success isn't always defined by intellectual prowess either, and many a successful individual could reasonably be defined as having a low IQ.

    8 points? Meh I can handle that.

  • rate this

    Comment number 26.

    14.Terry B
    Some of the most intelligent people I have known have enjoyed the odd spliff and tend to be much more pleasant people than those who don’t partake.



    Because I've found the opposite. The two regular users I know well, have become withdrawn, bad tempered and less able to hold down jobs & relationships, over the years I have known them.

    Mugs game.

  • rate this

    Comment number 25.

    I don't smoke cannabis. I have no idea why it's banned though, it seems so much better than alcohol or tobacco. There are occasional studies like this which appear to be designed to provide flimsy evidence to keep it illegal.

    Does anyone have any idea why? Is it because we could grow it at home which makes it very difficult to tax? I genuinely don't see why it isn't legal.

  • rate this

    Comment number 24.

    I have an IQ of 158 (tested twice, same outcome both times)

    I smoke LOOOOOOTS of cannabis (more than most of you can imagine)

    My opinion: Cannabis is the greatest substance on earth, and the people conducting this "experiment" were biased at the start, making their "findings" meaningless.

  • rate this

    Comment number 23.

    1000 people is a very small sample, especially when you make such drastic headlines from the result. How can you be sure the 1000 people used in this would have gone on to have a "high IQ"
    The IQ test is an incredibly out dated way of testing "intelligence".
    Having said that, I have used cannabis habitually since the age of 15 (now 31) and at the last test have an IQ of 140.

  • rate this

    Comment number 22.

    the majority of my friends including me smoke weed regulary , we dont go binge drinking get in fights etc. oddly we are all very professional and have good jobs in management so i think this is greatly flawed ,weed like anything can be harmfull if abused, most people dont abuse it nor suffer any side affects. although the text book brigade will choose to differ

  • rate this

    Comment number 21.

    This may be so, but without it we would have never had the "Coronation Street Cake Sketch" :D

  • rate this

    Comment number 20.

    I'm sure watching TV talent shows 4+ times a week probably doesn't do wonders for your IQ, yet we're happy enough to let people keep doing that.

    Every vice has its risks; cannabis is an occasional vice of mine and so long as the risks are mine alone to bear I'm happy to keep doing it.

    A lot of drug research seems politically motivated anyway, I've stopped paying attention.

  • Comment number 19.

    This comment was removed because the moderators found it broke the house rules. Explain.

  • rate this

    Comment number 18.

    Another "expert" report that proves the author has no idea really.

  • rate this

    Comment number 17.

    "It is such a special study that I'm fairly confident that cannabis is safe for over-18 brains, but risky for under-18 brains."

    Is this the first tentative step towards Legalization and regulation.

  • rate this

    Comment number 16.

    Finally, at the age of 58, I have foubd out why I am thick; I smoked a few joints when I was 15. I wonder how much better my life would of been if I'd of just stuck to tobacco on those few occasions.

  • Comment number 15.

    All this user's posts have been removed.Why?


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