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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  • rate this
    +76

    Comment number 3.

    "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."

    So why can't it be regulated in the same way as alcohol and tobacco? This study seems to confirm that it's unsafe for children, but not for adults.

  • rate this
    +65

    Comment number 64.

    Excessive use of ANY Drug (Alcolol, Cannabis, etc) cannot be good for a teenager...However, having met quite a lot of 'Smokers' at Uni, I can tell you that the odd smoke doesn't seem to do any 'Noticable' damage, not unless you can pick holes in a few of their PhD thesis's! Legalise it and wrest control back off the unscrupulous drug dealers, just like we do with booze..The tax wouldn't go amiss!

  • rate this
    +58

    Comment number 20.

    I'm sure watching TV talent shows 4+ times a week probably doesn't do wonders for your IQ, yet we're happy enough to let people keep doing that.

    Every vice has its risks; cannabis is an occasional vice of mine and so long as the risks are mine alone to bear I'm happy to keep doing it.

    A lot of drug research seems politically motivated anyway, I've stopped paying attention.

  • rate this
    +49

    Comment number 25.

    I don't smoke cannabis. I have no idea why it's banned though, it seems so much better than alcohol or tobacco. There are occasional studies like this which appear to be designed to provide flimsy evidence to keep it illegal.

    Does anyone have any idea why? Is it because we could grow it at home which makes it very difficult to tax? I genuinely don't see why it isn't legal.

  • rate this
    +46

    Comment number 300.

    To me this is no revelation, nor any reason for cannabis to be criminalised. Using cannabis four times a week is bound to decrease your IQ, I'm sure any drug would in that quantity. I mean, can you imagine getting drunk four times a week? You'd be pretty stupid. Just like any drug, pot has its negatives and importantly it's POSITIVES. Freedom of choice is important.

 

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