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 534.

    "Though it actually raises an more interesting question. If one drug is reducing IQ could some others be discovered to increase it?"

    Yes its called alcohol, my IQ goes through the roof on the stuff !

  • rate this

    Comment number 533.

    @ 518 Phosgene.

    No I am not a cannabis user, don't smoke or drink. I have quite a high IQ, thank you very much. If you read the comment properly you will see [1] that the comma separates two phrases making up the sentence, so the need for a "fullstop" and "It" is redundant.
    [2] I have not ended a sentence with a hyphen, it replaces a further comma with a phrase qualifying the previous phrases.

  • rate this

    Comment number 532.

    As pot is still illegal most users tend to go quietly stupid in their own space and time. Alcohol also makes people stupid, destroys brain function and has knock on affects like liver disease, marital breakdown and fatal accidents: not to mention all the public nuisance and mayhem it causes. Pot is a smug drug and stoners can come across as pretty silly sometimes. Drunks are just a scourge!

  • rate this

    Comment number 531.

    I've never used alcohol in my life - but I regularly imbibe cannabis to relax.

    Every weekend down the town where I stay I see aggressive, loud, alcohol users fighting and urinating on the streets.

    Some of them are so drugged up they can barely stand or speak.

    Most of them repeat this behaviour on a weekly basis, some are addicts & ingest the drug every other day.

    Someone should do something.

  • rate this

    Comment number 530.

    Chicken and egg, you probably have to have a low IQ to start with to use mind altering drugs.
    There goes a man who knows nothing about the subject at hand
    Please, feel free to get it deleted once more. I get deleted for saying he knows nothing about what he's arguing, yet he gets too call all drug users thick? get a life mods.

  • rate this

    Comment number 529.

    Erm no - you can obtain drugs just fine currently or you wouldn't be taking them. So why does anything need to change? I'm not infringing on your lifestyle by being against drugs and their legalisation. I'm entitled to my opinion same as you are entitled to your pro-drug opinion. Why do you have such a problem with that?

  • rate this

    Comment number 528.

    I'm starting to think that some of the anti-weed folk are basing their perceptions of smokers on that 10 minutes of "Dude, Where's My Car" that they saw once, rather than on any real life experience.

  • Comment number 527.

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

  • rate this

    Comment number 526.

    Question is, would a smoke help Frances Maude to say something sane?

  • rate this

    Comment number 525.

    Re:291 Leedsone. You have a family and break the law by using banned substances. So...not the perfect role model for your children....... Any one who uses prohibited substances is breaking the law and risking their health so maybe they have a low IQ in the first place.

  • rate this

    Comment number 524.

    It's a popular misconception that cannabis stimulates creative / critical thought ..."

    It's not a misconception, its a fact. Perhaps not in everyone, but certainly in most.

  • rate this

    Comment number 523.

    Another reason to discourage the use of cannabis not only deteriorating physical health, mental health but also a social status as well.

  • rate this

    Comment number 522.

    I love all these comments about how we should feel sorry for drug users and how bad their lives must be ! Smoking a little week socially with friends is no different to having a few beers and is very enjoyable. It doesn't make you a down and out druggy for god sake !! The only difference between a little smoke and a few beers is this Govt. deem one to be fine and the other not! Double Standards!!

  • rate this

    Comment number 521.

    @511. Statistician

    Your post doesn't make any sense, and then you resort to a childish 'lay off the drugs' jibe. Either you believe in infringing the freedoms of others (who in this case, choose to use cannabis), or you respect the lifestyle choices of others, as long as they aren't harming you.

  • rate this

    Comment number 520.

    Forget about all this cannabis drivel - I want to what's involved in a third eye squeegie (@494)! Sounds much more interesting

  • rate this

    Comment number 519.

    Dont fink it does

  • rate this

    Comment number 518.

    491. MoominMama
    "Looking at the spellings and grammar of a lot of the pro-cannabis comments, [1] looks like there's no need for the research -[2] it's self evident."

    It's self-evident that you are a cannabis user, or have a low IQ. Here is why:
    [1] This should be a full stop, followed by "It"
    [2] You have ended a sentence with a hyphen.

    It's a bit remedial. Rehab time?

  • rate this

    Comment number 517.

    Wouldn't you like to see a positive cannabis story on the news?
    Wouldn't you like to base your decision on balanced information rather than scare tactics, propaganda, and superstition?
    Wouldn't that be interesting?
    Just for once?

    There's a reason why many sane, functioning, moral, good, stable people; people like your neighbours, your kids, your parents take drugs every day...

  • rate this

    Comment number 516.

    Id like the same research done on:

    Exposure to TV, pop music & other media
    Reading the Daily mail
    Making comments on BBC HYS

    But above all - a study on those who have to live with the kind of people who want to make things illegal because they personally don't like it.

    I bet that really kills the neuron & synaptic response times.

    Arrogant lot.

  • rate this

    Comment number 515.

    Legalize cannabis. People have caused more hurt & harm than weed has ever done.


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