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"

Related Stories

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.

 

More on This Story

Related Stories

The BBC is not responsible for the content of external Internet sites

Comments

This entry is now closed for comments

Jump to comments pagination
 
  • rate this
    +3

    Comment number 711.

    Like all studies, this just gives a picture, and it doesn't say what kind of use? How many four times a week? When I was first pregnant, I was told it was ok to smoke up to 10 tobacco cigarettes a day. My friend's two daughters have PhDs in spite of being users since adolescence. And so the debate goes on .............

  • rate this
    +1

    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
    +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.

  • rate this
    +43

    Comment number 294.

    We do like to see things as all good or all bad, don't we? So, here we have something else that seems to do little harm in moderation (maybe even some good) but has some nasty results if we overdo it. A bit like chocolate, paracetamol, red wine, carrots, tuna, coffee, even water. You'd think we would learn quicker that things are very rarely all terrible or all marvellous!

  • rate this
    -17

    Comment number 291.

    I'm 26 and been smoking weed since I was 17 and I can say there nothing wrong with my brain I'm hard working and have a house and a family of my own and bet I'm smarter than half of the people that leave comments on here. Everyone is different and for some one to get paranoid when smoking weed means you must be paranoid already or have something wrong with you. Moan something else like alcohol.

 

Comments 5 of 6

 

More Health stories

RSS

Features

  • Shiny bootsMarching orders

    Where does the phrase 'boots on the ground' come from?


  • Almaz cleaning floorAlmaz's prison

    Beaten and raped - the story of an African servant in Saudi Arabia


  • Train drawn by Jonathan Backhouse, 1825The first trainspotter

    Did this drawing mark the start of a misunderstood hobby?


  • MarijuanaHigh tech

    The start-ups hoping to transform the marijuana industry


  • Child eating ice creamTooth top tips

    Experts on ways to encourage children to look after their teeth


BBC © 2014 The BBC is not responsible for the content of external sites. Read more.

This page is best viewed in an up-to-date web browser with style sheets (CSS) enabled. While you will be able to view the content of this page in your current browser, you will not be able to get the full visual experience. Please consider upgrading your browser software or enabling style sheets (CSS) if you are able to do so.