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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  • Comment number 334.

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

  • rate this

    Comment number 333.

    I also think most haters on here probably haven't read a single study on the benefits of marijuana. Have you?......HAVE YOU?.........if not then you know who you are. Just get on with it and open your eyes and leave us smokers alone. Any way i'm going to go back to my smoke now and read a nice book from the bookshelf i made myself when i was high last September. Solid oak baby, and engraved too.

  • rate this

    Comment number 332.

    Its great to see the legalise lobby so vocal. If you say 'legalise' you get plus ratings. If we legalise it, it will become even more available (not that its too hard now) but you'll be sending the message out to kids that's its OK to use when this report clearly says its not. Unless a low IQ is what you're after. Or Psychosis.

  • rate this

    Comment number 331.

    This doesn't seem to be conclusive research.

    1. "Intelligence" is poorly defined, so an IQ is a gimmick.
    2. IQ has already been shown to naturally downlevel with age only.
    3. I had an IQ of 160+ before I started. 152 is still nice.
    4. My best work was done during my "consuming" years.
    5. Exercising my memory in the intoxicated state, has proven to me benefits in mnemonics while not intoxicated.

  • rate this

    Comment number 330.

    Cannabis Sativa (Marijuana) is allowed in Hawaii County, Hawaii, USA, the citizens had some' voice in their political system. The resident can own up to 24 plants or 24 ounces. There are many substances that are banded and the reason is simply the people think in different ways, react differently and unsubdued by their government who formed a world for them to live in. Slaves are not feared.

  • rate this

    Comment number 329.


    what exactly is the point you are trying to make? Why on earth would a non-mathematician or scientist need to know about logarithms.
    Anyway I expect you'll be telling us you work it out in your head using mental arithmetic.
    How are you on musical theory? Do you have perfect pitch? Equally irrelevant questions.

  • rate this

    Comment number 328.

    I don't understand why so many commentators on the BBC are always so agressively pro-cannabis. "Alcohol is worse" and similar arguments are just terrible. Personally I'm all for there being one less legal drug around and the sooner the idiots who think cannabis is great grow up, the better!

  • rate this

    Comment number 327.

    316 Priestley20
    "Marijuana does NOT pose a threat to the general public. Marijuana is very much a danger to the oil companies, alcohol, tobacco industries and a large number of chemical corporations. Various big businesses, with plenty of money and influence, have suppressed the truth from the people."

    And some say cannabis doesn't cause paranoia...

  • rate this

    Comment number 326.

    This is the number two story like this is big news, it's been widely know for a long time that cannabis is very bad for young people as their brain is still developing (as is every other drug like tobacco, alcohol ect).
    What this isn’t addressing is how it effects adults, which is still an area of scientific uncertainty.

  • rate this

    Comment number 325.

    For years cannabis was the legal drug of choice for a weekend smoke by the middle classes. Now we know why there are such failings in the organisations needed by society - the people who deemed themselves the best, politicians and all, were having their brains addled by a weed.

  • rate this

    Comment number 324.

    These IQ tests are nonsense to begin with...
    Translation : if the study showed cannabis use gave higher scores in IQ tests then I'd brag about it at every opportunity

  • rate this

    Comment number 323.

    anything that is overdone is not going to be good for you, in moderation it is fine!!

    freedom of choice! regulate & tax all drugs and let people have a choice.....oh wait there is more money in tobacco and alcohol.............................

  • rate this

    Comment number 322.

    Total number of deaths from using cannabis = 0!
    Anybody who says cannabis smokers are not as clever or less motivated should really check their own IQ. I personally and many friends/colleagues have a joint after work and many of us are at the top of our respective fields. Its the same as anyone having a glass of wine or a cold beer when they get in from work. Legalise it and tax it.

  • rate this

    Comment number 321.

    Would like to see the stats to confirm - but if you start with a sample of 1000 children, then your number of general users is far less, and your number of chronic users is tiny.

    IQ is an poor measure of intelligence - widely affected by culture, class, ethnicity etc

    I do appreciate the validity of the overall multidisciplinary study, but don't recommend you read to much into this one aspect.

  • rate this

    Comment number 320.

    Lots of points been made here, most of them irrelevant.

    The key issue is 'Is weed more harmful than tobbacco/alcohol in the short-term and long-term?'
    The answer is no and no so legalise it and let the individual decide - so easy!

  • rate this

    Comment number 319.

    Were the children who were studied taking cannabis as part of a joint in cigarettes? Was the effect of tobacco studied? Also if cannabis has a potential loss of 8 IQ points, what is the effect of alcohol? One would make the presumption that regular alcohol abuse must lead to a deficit of 30-40 IQ points. Anyway, completely drug free, meat free and car free is the obvious way to go for good health.

  • rate this

    Comment number 318.

    @freethinker - Anecdotal evidence is not evidence. If you had paid attention at uni, you would know that.

  • rate this

    Comment number 317.

    Whether or not you personally agree with cannabis use or not is irrelevant. What needs to be understood is that problems associated with cannabis use (funding criminal gangs, users with a history of mental health problems, and underage use) can actually be more effectively dealt with by legalising and regulating. Also, who are you to to judge and prohibit another adult's peaceful activity?

  • rate this

    Comment number 316.

    Marijuana does NOT pose a threat to the general public. Marijuana is very much a danger to the oil companies, alcohol, tobacco industries and a large number of chemical corporations. Various big businesses, with plenty of money and influence, have suppressed the truth from the people.

  • rate this

    Comment number 315.


    I kind of agree. I have been a Cannibis smoker for approx 8 Years now, Starting back when i was just mid-way through my first year in GCSE. knowing it would affect my attention span i stopped 2 months before finals and revised, and passed with flyin colours, same goes with all rest of exams after, i am now workin in IT after completing my Computer science degree with a 2.1 hon's.


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