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 254.

    Having smoked weed for many years one of my regular past times was watching The Simpsons. I found this a source of great advice and encouragement:

    Ralph Wiggum - "smokers are jokers"
    Todd Flanders - "users are losers"

    Great words fellas. Thanks.

  • rate this

    Comment number 253.

    192. dbgvwwz
    'Surely you have to have a pretty damn low IQ to use cannabis in the first place'

    Or make trite comments like that.

  • rate this

    Comment number 252.

    Look, to some of you who don't now much about cannabis, here the possible factors -
    1. Your own brain produces natural Cannabinoids , the chemical bond found in THC.
    2. No, Cannabis does not cause lung cancer, but does excretes more tar, only if you smoke tobacco with it.
    3. For those if you who think cannabis is BAD, then research before you argue, school boy error.

  • rate this

    Comment number 251.

    173.Balloon Rake
    Only idiots make postings like this in the first place. Don't make me laugh about the "leads to hard drugs" joke. That was debunked years ago.

    Sorry I said I wouldn't stoop to insults but if we're talking about low IQ then - it's new low score player #2

  • rate this

    Comment number 250.

    @130 . They have a study that shows Cannabis is harmful to your IQ. Mind they is also evidence it can be better used than it is. Both for personal usage (chronic pan) and for the use of everyday items, such as paper, thus saving the rain-forest. However no study to show that religion helps with your fears or even evidence iy exists. Many people on here will also read the Daily Mail, enough said.

  • rate this

    Comment number 249.

    marijuana doesnt make people score lower on iq tests, we have seen highly intelligent intellectuals scoring low on iq tests yet they go on to become the foremost thinkers in their fields. so i would doubt the test before i doubt the person or in this case the plant. scare tactics to promote the prohibition of a natural earth resourse. no money to be made by big pharma so "marijuana is bad. m'kay"

  • rate this

    Comment number 248.

    Cannabis has not got stronger, the active ingredient in cannabis is THC, this is stripped from plants to make a solid or wet substance (hash and oil) or smoked immediately (green)

    2 million die per year from alcohol abuse, no one has ever died from Skunk, hash or weed. It may decrease our IQ, but IQ decreases over time for a host of different reasons.

    Another inaccurate, Daily Mail article #yawn

  • rate this

    Comment number 247.

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

  • rate this

    Comment number 246.

    So if you are blonde and a dope user , you really are giving yourself a mountain to climb ........or one hell of a high cloud to come down from , whichever is most suitable

  • rate this

    Comment number 245.

    Cannabis is addictive and buying it funds crime, my friend has smoked it from teenage years and his usage now is pretty high. Without it he is bad tempered and anxious. It is time for it to be legalized, controlled and taxed, take away the coolness and hopefully some kids may not start it. In fact legalize all narcotics and the crime rate will halve.

  • rate this

    Comment number 244.

    Dude! Where's my car?!

    Where's your car dude?

  • rate this

    Comment number 243.

    My mind is so active during most of the day and I find it difficult to shut off. If someone uses a prescription drug regularly for this purpose then that's fine but using 'weed' makes you dumber than the proverbial? I think there needs to be wider recognition of the addition potential but ocassional use is of no detriment whatever.
    I love the puritanical musings of the just say no mob.

  • rate this

    Comment number 242.

    Perhaps the cannabis smokers weren't really bothered about how well they performed in an IQ test?

  • rate this

    Comment number 241.


    Stereotypes are often grounded in fact & who can't remember meeting real life versions of 'Neil the Hippy'?
    I have never met a"Neil the Hippy". Which sterotypes are grounded in truth then?
    Laxy mexicans? Dizzy Blondes? Thieving scousers? Tight Scots?

  • rate this

    Comment number 240.

    Squirrel couldn't have put it better...
    I worry far more about pervasive pop culture, teeney media and other forms of commercial and corporate brainswashing shrinking the common knowledge base and stunting young peoples ability to think as individuals in an abstract manner more than a naturally occuring drug with no known (or medically defined) toxic dose. Tea anyone????

  • rate this

    Comment number 239.

    216. Controlled Pair

    "I never impair my senses. I'm always sharp as a tack, just in case trouble comes my way."

    And they say dope smokers are paranoid!

  • rate this

    Comment number 238.

    "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."

    This probably the most revealing statement fromt he article.

    For all of you generalising, get a grip, your point has no merit. Cannabis users come from all backgrounds/origins. It's like saying everyone who drinks is an alcoholic. Some people's views make me despair

  • rate this

    Comment number 237.

    I cannot help but agree with this, as I openly admit 14-20 I smoked a lot of weed, and while much can be put down to the fact I paid a lot less attention in class and was present less due to the fact I was off getting high, even after not doing it for years I FEEL that I have nuked some of my potential, that my brain is slower to grasp concepts then when I was young.

    Short term memory sucks too.

  • rate this

    Comment number 236.

    @208 philuk2000

    One could argue that the taxation sought to combat the criminal activity of drug dealers is considerably greater and that legalisation would result in a net tax gain. It's a little ironic that your referred to Delphi -- what with her sitting on a mind altering rock bed all day long and issuing wisdoms to the world leaders. I though it amusing anyway.

  • rate this

    Comment number 235.

    It is perfectly fine for Cannabis Smokers to have a lower IQ, how would we have a reasonable spread of thicko's and smarto's without them?

    We need the Dumb ones to make up the average!


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