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 474.

    It still shocks me too this day, that as a "free" country,that our people will still try too take away the freedoms from others, just because they don't like them.

    The only societal harm around cannabis comes from drug gangs, take it away from them, and control it.
    Thereby eliminating the under 18's from the market anyway. Thereby making this "research" irrelevant.

  • Comment number 473.

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

  • rate this

    Comment number 472.

    I could ask the same question of the people saying that I'm boring just because I don't take drugs. I'm not ashamed of standing up against drugs - I don't view that as forcing my lifestyle on anyone. I'm just stating my position same as the pro drug people are. You have a problem with people having a different perspective from yours?

  • rate this

    Comment number 471.

    Clearly cannabis is dangerous - as are tobacco and alcohol.

    Occasional use may be OK if you are lucky, but regular use can be as damaging as tobacco.

    Much better to legalise it - put a high tax on to discourage use, but do not criminalise. (Unlicensed dealers would be illegal however, along with illicit dealers in other drugs.)

  • rate this

    Comment number 470.

    "enjoy life."
    Fixed that last sentence for ya. No need to thank me.

  • rate this

    Comment number 469.

    I am slightly surprised that smoking 4 times ("at least") a week for 20 years+ only causes a loss of 8 points. A little less and I doubt it would have been statistically significant.

    Proff Moffitt is quite clear that any loss in IQ from those who start over 18 can be recouped. Those who start as teenagers don't recover "fully".

    I wonder if the same is true for booze? I doubt it.

  • rate this

    Comment number 468.

    @ Leedsone.
    You may feel it has not affected your brain( and you may well be right) however it is not claiming smoking that young makes you stupid,it is saying it limits your chances of reaching your full intellectual potential.So while you are a fine specimen of homo-sapiens you'll never know if you could have been our next Einstein ,Hitchens or Virginia Woolf

  • rate this

    Comment number 467.

    I used to smoke pot. Now I'm at the top management of an oil and gas service company.

  • rate this

    Comment number 466.

    Either we treat anything remotely 'unhealthy' (fast food, booze, cigarettes, contact sports and a million other things) the same by banning or denormalising them OR we regulate them, educate and promote moderation but accept that what people choose to do with their own bodies is ultimately their own business. Islington trendies who smoke pot but would outlaw my Friday night takeaway take note.

  • rate this

    Comment number 465.

    What nonsense! As a member of a branded intellectual group, my IQ is fine. For 25+ years i have been a heavy cannabis smoker.
    Look at the people who "continue" to smoke cannabis, at least 70% are of low intellect or facing disadvantages to begin with.
    Smokers do not achieve due to lack of motivation - not lowering of the IQ. If you dont use your brain...
    Use drugs, dont abuse drugs, enjoy life.

  • rate this

    Comment number 464.

    For some reason this seems to be a particularly emotionally evocative subject with those who smoke cannabis entrenched by the belief that it is a relatively harmless substance, especially when compared to the larger impact of alcohol on society. On the other hand you have the more law abiding citizen who would be as violently opposed to alcohol were it illegal as they currently are opposed to weed

  • rate this

    Comment number 463.

    Anyone with a significant IQ wouldn't use any proscribed drug anyway!

    You mean like these low-IQ wasters and idiots (i.e visionaries, artists, cultural, social, intellectual and political giants)?

  • rate this

    Comment number 462.

    Any substance either alcohol prescription drugs tobacco or illegal drugs effects your body in a negative way with either short term or long term damage.Some people have more tolerance for substances compaired to others, so the way it effects one person who many have a bad reaction first time to a certain substance and others not.

  • rate this

    Comment number 461.

    PS. There was another 'survey' a few years ago (with similar low reliability) which showed conclusively that Cannabis PROTECTS the user from Lung Cancer and other smoking related illnesses.

    It showed that there were far fewer deaths from Lung Cancer among cannabis users than you would expect, bearing in mind that they are consuming tobacco as well!!

  • rate this

    Comment number 460.

    Absolute twaddle and terrible reporting on an inaccurate study. Where's the control? this screams of the Nixon era with the Rhesus Monkeys all over again... I'd like to mention here that since I've been smoking weed pretty much every day since the age of 16 that I've seen my own IQ fluxuate up and down between 128 and 139.
    Which generally puts me in the top 5% of the population.

  • rate this

    Comment number 459.

    So stoners are less likely to achieve things? Revelation! I've never been a dope smoker, despite my stoner-like musical preferences. My mates at Uni used to sneak off for a joint which I thought was a bit lame.One of them had to resit his 1st year, and another friend got kicked out. Both really gifted people . Each to their own, but I don't want to see it in Tescos. It ain't good for society man!

  • rate this

    Comment number 458.

    I don't smoke cannabis. However, I did get drunk on two occasions this bank holiday weekend, Friday and Sunday. I don't feel great today...pretty sure my IQ has dropped.

  • rate this

    Comment number 457.

    "297. fraz3375

    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."

    Not off the top of my head, but could you tell me the role of Hjc and RadA proteins during homologous DNA recombination off the top of yours?

    So, a daft point, hey?

  • rate this

    Comment number 456.

    I find this very credible.

    Of course, there will always be some exceptions to any statistical 'rule'. Perhaps some older people might be similarly vulnerable, and some younger not be.

    However, I can easily believe that generally heavy use of dope, particularly during teenage years, tends to make people 'dopey', for want of a better term.

  • rate this

    Comment number 455.

    @ 446. Statistician

    That's great, but why do you feel you need to force your own lifestyle choice onto others?


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