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 514.

    I'm 20. Never have and never will touch cigarettes or drugs but I do enjoy the odd pint now and again. I think people will do as they please regardless of government warnings and advertisements. We often forget there are positives in everything; you can't have negatives without positives.

  • rate this

    Comment number 513.

    Overdoing anything isn't the only thing that's bad for you - so is reliance on an outdated inappropriate measure like IQ to measure anyone's success in life.

  • Comment number 512.

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

  • rate this

    Comment number 511.

    What? :-s Who said anything about banning anything that isn't already illegal. I think the drugs may be causing you to become a bit paranoid. You maybe should lay off them for a bit (no I'm not forcing you too - I just think addiction is a terrible thing).

  • rate this

    Comment number 510.

    1 Minute Ago
    Looking at the spellings and grammar of a lot of the pro-cannabis comments, looks like there's no need for the research - it's self evident.

    How odd, I was about to remark about the poor spelling and grammar by the pro lobby. It would seem that we have identified another irrelevancy. Next please!

  • rate this

    Comment number 509.

    Well it's not called 'dope' because it doesn't make you dopey, but an 8 point fall seems small in a total population where the range of IQs is very much wider. How much more would those who had an 8 point fall be realistically expected to achieve if they had received a legal anti-depressant/anti-anger regime? Odd that we never hear Why people start, or why the word 'no' might magic the need away.

  • rate this

    Comment number 508.

    @fraz3375 it's such a pity that you've shone a light on your stupidity in believing that mathematical knowledge somehow relates to intellect.

    You've also highlighted your own lack of knowledge in maths by not even presenting a problem as 10^x in base 2 (aka binary) would be 1010^x as you have not given a value for the exponent 0xA^x if youd like it in base 16 (Hexidecimal)

  • rate this

    Comment number 507.

    Chicken and egg, you probably have to have a low IQ to start with to use mind altering drugs. The decline could be natural in a group of the stupid.

    Though it actually raises an more interesting question. If one drug is reducing IQ could some others be discovered to increase it?

  • rate this

    Comment number 506.

    Oscar Wilde, considered a great writer, smoked copious amounts of opium. As did Edgar Allen Poe and numerous other writers around this time period. Bit more extreme than cannabis, what does that tell you?

  • rate this

    Comment number 505.

    439.Karajishi "I have had a state of mind that encouraged me to question the world around me...a huge factor of it's original control in the US".
    I was under the impression that fearing competition, the paper and cotton barons lobbied Congress for its prohibition. It's a popular misconception that cannabis stimulates creative / critical thought but ultimately there's a reason it's known as dope.

  • rate this

    Comment number 504.

    I'd be interested to see what a similar research would find on binge drinking...

  • rate this

    Comment number 503.

    It is widely known within the academic world that drugs have been in use for thousands of years there is a variety of different reasons why people take drugs, not just to escape things. =Why have wine with your food if all you used drugs (and alcohol is a drug, just one that is convenient for the government to tax) for is to escape? You wouldn't blame the alcohol if some kid goes and get plastered

  • rate this

    Comment number 502.

    You might as well say the same about alcohol. while the body is still growing and developing, any mind altering substance is going to have an adverse effect. Even for adults, continually overdoing it is going to lead to irreversable adverse effects. And all it took to reach that conclusion, was a bit of common sense.

  • rate this

    Comment number 501.

    Just for the record, Queen Victoria was a part-time stoner, for period pains. As were many at the time. It's use only fell out of favour when faster acting alternatives came along. No idea what her IQ was, but in relation to society as at was then I think she'd be regarded as a success.

  • rate this

    Comment number 500.

    I smoked cannabis regularly as a young teen and a young adult. When I took the GRE (standardized exam for entrance into graduate school in USA) I scored 97% percentile on the analytical version (i.e. 97% people who take the exam scored lower than me). I believe I even smoked cannabis the night before the exam. Generalities don't apply to everyone, even beyond this study..

  • rate this

    Comment number 499.

    The health benefits of this plant when used properly far outweigh the negatives these studies keep conjuring up. When taken in it's raw state, unheated.. It is NOT psychotropic, and therefore does NOT adversely affect the brain, in fact it boosts neuron production and has been shown to help people with Alzheimer's and dementia. The biggest positive is the THC however which attacks cancer cells.

  • rate this

    Comment number 498.

    J Workerbee
    ...we should feel sorry for them that their lives must be pretty bad to have to use drugs in the first place."

    Thanks for your concern, but no need to feel sorry for me. I've got a great job as a genetic researcher and a wonderful partner that I am getting married to next year. Broadly speaking, life couldn't get any better!

    Though sparking up a fatty later might do it! ;)

  • rate this

    Comment number 497.

    Don't need intelligence these days.... Just be stupid, loud and very chatty, as long as you have a few big words to use occasionally, people will automatically assume your intelligent.... And stupid people automatically like other stupid people.

    True intelligence is often a curse, in life, in work and in general.... So the idea that cannabis lowers your IQ, is really a moot point.

  • rate this

    Comment number 496.

    @472: Statistician: Well if you're 'standing up against drugs' equates to supporting prohibition, then how are you not forcing your lifestyle choice on anyone? I suspect the comment saying you were 'boring for not taking drugs' was tongue-in-cheek. I don't have a problem with people having a different perspective to mine, but I do when they support a ban on another adults' peaceful activity.

  • rate this

    Comment number 495.

    To everyone here who disagrees with cannabis use and uses the study to confirm that belief.. try reading on.. The study also clearly says there is no problem for over 18's.

    So what is it then.. Either you accept the study or you dont..Ah no.. you want it BOTH ways.. pick out the bad parts and play down the good. Remind me not to ask you to do anything that requires you to be objective.


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