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 274.

    Having smoked cannabis for many, many years on and off i actually find that i become MORE organised and my brain seems to function a lot better when i am on it. Maybe it is because i have A.D.H.D? who knows, all i know is that i have managed to raise 2 children, hold down a decent job and achieve lots of my goals. I also still have the 131 score i had when i was 18.

  • rate this

    Comment number 273.

    As with much in life, "Everything in moderation".
    Over-doing anything is bad for you, cannabis is no different.

  • rate this

    Comment number 272.

    not many evil ones get into the pot boys little get togethers-maybe if the lunitic adolph had have been hanging out with the austrian da da's smoking a joint and on the pull instead of being a tight a*sed nondrinking veggie,the nuremberg rally may have turned out to be the first hippy parade in history,instead of dropping steal it could have been flowers.

  • rate this

    Comment number 271.

    If you do anything in excess it's going to mess around with your brain. This 'study' is for excessive use..On a lighter note..
    I like the Family Guy skit to the tune of 'The Old Bamboo'
    ...A bag of weed, a bag of weed..etc sung by Brian and Stewie

  • rate this

    Comment number 270.

    While likely significant to a statistician, eight points of I.Q. isn't that much to an individual. I'd think mine has eroded at least that much over the last fifty years, and I haven't smoked anything. And since when is there an automatic tie-in between I.Q. and achievement? If there's a predictor of success in young people, I believe it's in attention to detail. Stoners lose there.

  • rate this

    Comment number 269.

    This is a really sloppy bit of journalism BBC, you’ve cherry picked negative data from the report that’s being peddled by the anti lobby and ignored other key points, you also wrongly stated it increased the number of cases of schizophrenia which it doesn’t and this is stated in the report, in fact if you Google DCD cannabis you’ll see it’s being used to combat it.

  • rate this

    Comment number 268.

    why is the phrase "naturally occurring" always used to bolster an argument that something isn't harmful? Cliffs are naturally occurring but jumping off one is probably a bad idea
    But people are allowed to hang glide and abseil legally all the time.

  • rate this

    Comment number 267.

    @241 Foetus Interruptus


    Stereotypes are often grounded in fact & who can't remember meeting real life versions of 'Neil the Hippy'?
    I have never met a "Neil the Hippy". Which sterotypes are grounded in truth then?
    LOL, I've met quite a few & if you had read my entry properly, you would have known I was referring to 'Punch Drunk Boxers' who also suffer from brain damage.

  • rate this

    Comment number 266.

    I believe people have the right to abuse their bodies in whatever way they see fit. My only objection is that by doing such things they harm no one else or waste the time and resources of others directly or indirectly..
    See it's not that difficult to work out.

  • rate this

    Comment number 265.

    HOG WASH Cannabis Sativa (Marijuana) is psyche active. What you are seeing is a natural instinctive response to a propaganda world used to destroy common sense. The reason nations by majority do not like the people using it is because the chemical make up of their citizens make them stunned, fear frozen, unable to think and predictable. The IQ answers are their truth, not governed programming.

  • rate this

    Comment number 264.

    Whilst I believe there is probably a drop in IQ scores, I also believe that cannabis is exceptionally good at increasing your creativity and imagination for painting a picture or writing a song etc so there is a balance, not to mention all the other positives, medicinal and environmental.

    Im pretty sure iv never seen a "stoner" laying in the street or fighting up town on a Saturday night.

  • rate this

    Comment number 263.

    And_here_we_go_again #61
    "There have been studies done that suggest that people with higher IQs are actually more likley to take drugs "

    Assuming that blog fairly represents the research (it does not link to the original article), such studies say nothing about the reliability of todays research findings, which say that, on average, cannabis makes the intelligent less so, and the thick more so.

  • rate this

    Comment number 262.

    So people are made stupid by smoking canabis - Ahh no thats not what is being said at all - Children who smoke canabis do worse in a blooming awful measurement known as an IQ test - Which only measures a single ability and is useless at measuring a person as a whole. IQ tests need to be discredited.

  • rate this

    Comment number 261.

    @192 "Surely you have to have a pretty damn low IQ to use cannabis in the first place"
    Really? Your comment is a pure prejudice that have no basis in science. It's a drug that is significantly less harmful than alcohol, many over the counter medicines, caffeine etc. so what would be a really educated and smart choice in your opinion?

  • rate this

    Comment number 260.

    There's a reason it's called dope. Duh.

  • rate this

    Comment number 259.

    why is the phrase "naturally occurring" always used to bolster an argument that something isn't harmful? Cliffs are naturally occurring but jumping off one is probably a bad idea

  • rate this

    Comment number 258.

    Nobody who knows long term cannabis users will be surprised by this research.

  • rate this

    Comment number 257.

    interesting how cannabis and tabacco smokers react in the same defensive way to any suggestion it could be harmful, while the non smokers can see the evidence anyway. Still, as they say, it's their choice.

  • Comment number 256.

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

  • rate this

    Comment number 255.

    I've smoked a lot - hell of a lot. To be honest it's messed up some relationships and turned me into a bit of a recluse at times. But , I hardly drink, never been arrested, have a good job and earn ok money. The problem is folk who wake and bake, makes you go a bit crazy. As an occasional pastime, I think ok. Theres no clear cut answer, it is as bad as it is good. Freedom of choice is important


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