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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  • Comment number 714.

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

  • rate this

    Comment number 713.

    You can sit in a room full of people with the joints coming thick and fast and everyone will go home and say they've been getting stoned. Actually none of them has been smoking it: Hash is a very scary drug you have to really know what you are doing to smoke it - UNLESS you are drinking as well, in which case you needn't have bothered: People drink to cancel out the effects of the dope: Low IQ!!!

  • rate this

    Comment number 712.

    1. Even if 1 in 10 of the cohort became regular cannabis users, the sample of potheads is only 100. Small samples/bad science give erratic results. Straights who think their reality is the only reality are the ones who are *really* dangerous.
    2. Steve Jobs said Windows would have been a better operating system if Bill Gates had dropped acid. The shock of the true.

  • rate this

    Comment number 711.

    Like all studies, this just gives a picture, and it doesn't say what kind of use? How many four times a week? When I was first pregnant, I was told it was ok to smoke up to 10 tobacco cigarettes a day. My friend's two daughters have PhDs in spite of being users since adolescence. And so the debate goes on .............

  • Comment number 710.

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

  • rate this

    Comment number 709.

    And the Government and Society continue to allow this drug to be uncontrolled in anyway. In its current uncontrolled state this drug is easily available to most children. Its about time we control this drug and at least try to restrict its sale to children. Decriminalise it, and license sellers, so we can put some control on who gets these very easily available drugs.

  • rate this

    Comment number 708.

    I've been smoking cannabis for years and I still managed to pass my GSCE English, so there!

  • rate this

    Comment number 707.

    To continue: I am 58 years old and have enjoyed pot for 41 of those years. In that time I have accumulated four degrees and have a wonderful international career. I have met and smoked with some of the most amazing people from all over the world. I do not think anyone should partake of anything they are not comfortable with, but as for me, ıt's been great! Oh yes, my IQ score: 175.

  • rate this

    Comment number 706.

    @689 Arthur1958

    This is a robust study, but even if it were replicated and substantiated what aspect of the study would the government focus on? Here, in this debate, the focus is on the loss of IQ for developing brains, and not the fact that developed brains suffer no harm. A study is only as good as its use IMHO.

  • rate this

    Comment number 705.

    "Freedom of choice is important."

    Freedom of choice to be an unemployable zombie paid for by the taxpayer? A paranoid moron likely to sense non existent trouble?

    Freedom is a very elastic concept and isn't an excuse to allow deadbeats to drop out or drift reliant on others. That's before the crime and gang implications that cannabis brings.

  • rate this

    Comment number 704.

    Surely a fairer way of limiting demand for university places than by imposing excesive fees.

  • rate this

    Comment number 703.

    We are bombarded with alleged results produced through dubious methodologies and it is easy to “blow them off” but this was an amazing effort.

    Dubious at best, small study, no control group. How were the children chosen for this test? Were they all chosen from an underachiveing school with poor teachers? We don't know enough to say that the test meets standards yet

  • rate this

    Comment number 702.

    Abject nonsense. This trial was flawed and the results suspect. Ask any Kiwi about Dunedin and that will explain why. Most of the sample will have been related to each other and therefore it cannot be an across the board sample of young people.
    Official briefings tel lus that nicotine stays in the body fat twice as long as cannabis residue, so what does that do to IQ?

  • rate this

    Comment number 701.

    I seem to remeber,some stink in the year 2000,involving £100,on the spot fines for people in possesion,proposed by Ann Widdecombe.This was stopped by William Hague,when eight members of his Shadow Cabinet,admitted,to smoking when younger.

  • rate this

    Comment number 700.

    Smoking pot makes you stupid, causes psychosis, laziness, harms your lungs and a host of other problems. Medical marijuana is another invention of the pot smokers to indulge in their favorite pastime. In any event, about 10% of the population will keep on doing it, study or no study!

  • rate this

    Comment number 699.

    Don't want to pick threads but I'm fairly sure there is a significant proportion of those in government that have smoked a few spliffs in there time, only they now conveniently forget that being in the public eye. Well follow the consensus in these comments and you'll find that the public do actually support legalizing it so drop the mantra of 'ill effects' etc and treat it just like alcohol then.

  • rate this

    Comment number 698.

    Lower IQ , suppose that's why they call them dope heads.

  • rate this

    Comment number 697.

    The BBC should really change the misleading title on the front page.

  • rate this

    Comment number 696.

    so they do this study in a nation that has severe alcohol problems they do not state how much alcohol these same people consume so how do know its not the alcohol that has caused this effect?
    I know many people who smoke everyday and they all have various highly skilled jobs some of them are in the science fields as well so i think this study is hogwash

  • rate this

    Comment number 695.

    I guess this explains why the Labour party can't understand why a country should not try and borrow its way out of a recession borne from excessive debt.


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