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.

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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 314.

    I think you should worry about mental health problems as paranio and depression. Deal with addicts in this country give them hope a job and something to aim for.

  • rate this

    Comment number 313.

    Are people from New Zealand not all dopey to start with anyway?

    Only kidding mates!

  • rate this

    Comment number 312.

    If cannabis has no substantial effect on the human brain, why is it so easy to identify someone who does, or used to, smoke regularly within a few seconds of talking to them?

  • rate this

    Comment number 311.

    @260 fraz3375

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

    Of course there is. The etymology of the word is derived from the similarity of dipping sauce and liquid opium paste. Nothing to do with IQ I'm afraid. Words do undergo a change in use though, and clearly at some point someone decided that a thick sauce and a thick person were the same.

  • rate this

    Comment number 310.

    Robbo # 247:

    "Why is it so bad to have a low IQ? At least there will be less people about who think they know everything!"

    Trouble is, as these message boards regularly show, IQ and knowing everything so often seem to have an inverse relationship...

  • rate this

    Comment number 309.

    "280.Victor Mackey

    I can state it also robs them of their intelligence."

    And I can state it doesn't. Touché. Even if this report is true, IQ does not equal intelligence (see the Horizon programme).

  • rate this

    Comment number 308.

    275 Drunken Hobo
    AS I've said before, A burning plant matter produces an excess of TAR, THC as a natural substance in the bloodstream is fine, BUT, Tobacco smoke caries Benzopyrene, the chemical found in coal, tobacco and cars, that chemical attaches to the p53 gene - causing the cancer to grow
    please, if your going on about how bad it is - Research before you argue.
    Nuff said..

  • rate this

    Comment number 307.

    I started smoking cannabis regularly as an 18 year old, though i had experimented with other things earlier (Thus conflicting the gateway drug theory). I am a student at a top university, I know several people who started smoking daily at age 15-16, are doing more intellectually challenging subjects than me, and are getting 1st's in them. Not exactly unbiased, but it doesnt make it any less true.

  • rate this

    Comment number 306.

    Alcohol is probably the biggest threat to teenage brains surely?

  • rate this

    Comment number 305.

    It has it's uses it's used to treat glaucoma and some other medical conditions legally in some parts of the states,I used to use it regularly and held down a good job,never hurt or robbed anyone and functioned as a decent member of society,It will never be legalised here though as the Govt had a "drug tzar" who said it should be legal so they sacked him!

  • rate this

    Comment number 304.

    These IQ tests are nonsense to begin with, just look at the difference between the average kid from Germany or Hong Kong and the average kid from Nigeria or the DRC. They are subjective and biased nonsense and say more about the people designing the test than those being tested.

    And any change is likely to do with cannabis' effects on motivation rather than some chemical sea-change in the brain.

  • Comment number 303.

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

  • rate this

    Comment number 302.

    3 Minutes ago
    Shockingly-bad science reporting.

    A meta-analysis of their sub-tests results reveals that the headline assertion isn't statistically significant.

    That's a relief because other the results it provides would suggest cannabis use massively increases arithmetic and other abilities.


    Same is true of nicotine.

  • rate this

    Comment number 301.

    Isn't Alcohol much more damaging and dangerous? I agree with 264. Adrenalin that "stoners" do not get into fights and/or become overly aggressive, so shouldn't we be "proving" these well known facts of alcohol? which is ridiculously legal because. uh, well I'm not exactly sure why that is legal and cannabis isn't... but the law's the law I guess...

  • rate this

    Comment number 300.

    To me this is no revelation, nor any reason for cannabis to be criminalised. Using cannabis four times a week is bound to decrease your IQ, I'm sure any drug would in that quantity. I mean, can you imagine getting drunk four times a week? You'd be pretty stupid. Just like any drug, pot has its negatives and importantly it's POSITIVES. Freedom of choice is important.

  • rate this

    Comment number 299.

    Does that makes them gone to pot heads?

  • Comment number 298.

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

  • rate this

    Comment number 297.

    ok,had enough of the pro-dope lobby here. Is there any one of you who has been advocating it who can answer me this- what is 10^x in base 2?.I don't think one of you is capable of even understanding the question.

  • rate this

    Comment number 296.

    What always holds me in amazement are the rationalizations for using the stuff. Is life so bad that we cannot use our minds as they are to be happy, or content, or work our way out of a bad place? Some people seem not to want to try. Therein lies the real tragedy, as I see it.

  • rate this

    Comment number 295.

    there are no recorded deaths from use of cannabis alone.Cigarettes however kill more than cancer, aids, heart disease combined in U.S.A. Cannabis is one of the most useful medicinal resources on the PLANET and because it is free to grow and its counterpart hemp(also one of the most durable resource on the planet)its SOMEHOW illegal. i guess there is no money to be made for the pharma oligarchs....


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