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
    +2

    Comment number 74.

    'Young cannabis smokers run risk of lower IQ, report claims'

    D'oh!

  • rate this
    +10

    Comment number 73.

    Apparently it also reduces the ability to read. The finding applied to "individuals who started using cannabis in adolescence and then carried on using it for years". Many of us dabbled; many of us would like to see the law changed, and parts of this are useful ammunition. So if you dodged the bullet, good for you; but you won't invalidate a serious study just because you don't like its findings.

  • rate this
    0

    Comment number 72.

    Hmm interesting. BUT!
    Was it mixed with Tabacco and therefore could it have been tobacco that caused it? What amount was smoked and how long after each does was the IQ tested?

    There seems to be a lack of scientific fact missing from this story.

  • rate this
    +21

    Comment number 71.

    If you start drinking alcohol at age 14 as a recreational drug it will mess you up just as badly as cannabis would. The good news is that it has little effect on adults. Time to legalize and collect taxes on it I think, we could prevent thousands of alcohol related deaths every year if we did.

  • rate this
    -17

    Comment number 70.

    uh... uhhhhhhhh...

    man that's some gooood s..t

  • rate this
    +2

    Comment number 69.

    41.bentyger
    A, B: so we need to use different methods for different test subjects? can you think of a way to discredit a study more?
    C: this may be a prob.
    D: so basically pro-drugs will support, anti-drugs will oppose, what's the point of any study? Argue the merits.
    E: Tonnes of ppl won lottery, does that make it a good investment strategy? no, hence the study.

    Maybe you smoke too much.

  • rate this
    -2

    Comment number 68.

    Or maybe people with lower IQ's are more likely to smoke heavy amounts of cannabis in the first place.
    ---
    I'm a heavy smoker, please, take the pepsi challenge with me, I'll beat 80/90% of you non smokers, without breaking a sweat.

    And people accept these "surveys" as scientific fact. LMAO.
    To get the IQ of a group, take the IQ of the group, and divide by the number of people in said group.

  • rate this
    +3

    Comment number 67.

    "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."

    Why is an 18-year old brain so special? The brain doesn't reach maturity until the mid-20s.

  • rate this
    +2

    Comment number 66.

    I am sure Carl Sagan had a pathetic IQ.

  • rate this
    0

    Comment number 65.

    I'd love to see a similar study carried out today, now that we have the Skunk strains available.

    The cannabis available during the study wouldn't have been as strong as today's weed. What studies have been done around skunk indicate some worrying changes to teenage brains.

  • rate this
    +65

    Comment number 64.

    Excessive use of ANY Drug (Alcolol, Cannabis, etc) cannot be good for a teenager...However, having met quite a lot of 'Smokers' at Uni, I can tell you that the odd smoke doesn't seem to do any 'Noticable' damage, not unless you can pick holes in a few of their PhD thesis's! Legalise it and wrest control back off the unscrupulous drug dealers, just like we do with booze..The tax wouldn't go amiss!

  • rate this
    +1

    Comment number 63.

    I would like to say!!!!!!
    damn the munchies are here

  • rate this
    -6

    Comment number 62.

    I think if a person is stupid enough to smoke it in the first place, this research isn't really going to matter is it

  • rate this
    +25

    Comment number 61.

    There have been studies done that suggest that people with higher IQs are actually more likley to take drugs ( http://www.lapalomatreatment.com/blog/high-iq-and-drug-use-linked/ )

    This makes sense to me, in the same way that people with higher IQs are more likely to suffer depression, you're more aware of the world around you and hence more aware of it's faults.

  • rate this
    +8

    Comment number 60.

    51. Shandyanne
    All of us have IQ above 130. .... I am not saying that pot is good for you, but it doesn't make you stupid either. People around you does.
    ---
    People around you DO perhaps? And even that doesn't make much sense.

    My father-in-law smokes 60 a day (tobacco) and is 73. Does that prove tobacco doesn't cause cancer & heart disease too?

  • rate this
    +3

    Comment number 59.

    IQ is neither a completely accurate test for intelligence or a significant marker of achievement. So yes lets knock cannabis once more and forget that sanctioned drugs like alcohol really do have massively significant risks.

  • rate this
    0

    Comment number 58.

    Or maybe people with lower IQ's are more likely to smoke heavy amounts of cannabis in the first place. After all, if they're struggling with education and progressing into adulthood, then they're all the more likely to seek other forms of mental stimulation.

  • rate this
    -2

    Comment number 57.

    I have never tried nor intend to try cannibis however, I think this report is very flawed in it's findings and is money badly spent.

  • rate this
    0

    Comment number 56.

    can some one please tell me who sponsored this survey????

  • rate this
    +3

    Comment number 55.

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

    'Seem to'? Against what criteria? Is 'achievement' now hovering at a required level, with those failing to reach it regarded as not coming up to someone else's expectations? I thought it was subjective. Pressure to conform is immense in this society, underwritten by endless 'expert' reports.

 

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