Teen cannabis use may damage brain for life, warns major study

Fears that drug 'rewires' adolescent minds as scientists find persistent smoking reduces IQ

Teenagers who smoke cannabis regularly could be permanently damaging the development of their brain and are likely to end up with significantly lower IQ scores than teenagers who do not use the illicit drug, a major study has found.

People who started smoking cannabis as adolescents were found at the age of 38 to be still suffering from a drug habit they had started more than 20 years earlier, scientists said.

The findings will help to dispel the common belief among teenagers that cannabis is a harmless drug and will lend weight to calls for more to be done to prevent cannabis use among teenagers, the researchers said.

The study suggests that weekly cannabis use before the age of 18 results in an average decline in IQ score of eight points, which is enough to move someone of average intelligence into a category that is well below average.

Scientists said that the study is the first to show that cannabis use in adolescence – but not cannabis use that begins in adulthood – can cause a significant long-term decline in IQ that does not appear to be reversible when people stop using cannabis.

The researchers believe this is evidence that cannabis can interfere with the development of the adolescent brain, which continues to undergo neural growth and "rewiring" during early teenage years.

"Quitting or reducing cannabis use did not appear to fully restore intellectual functioning," said Madeline Meier, of Duke University in North Carolina. She was the lead author of the study published in Proceedings of the National Academy of Sciences. " IQ decline could not be explained by alcohol or other drug use or by reduced years of education among persistent cannabis users… marijuana is not harmless, particularly for adolescents," Dr Meier said. "Somebody who loses eight IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come." The study used data gathered from a cohort of 1,037 children born in 1972-73 in Dunedin, New Zealand. IQ tests carried out when they were 13 were compared with IQ tests completed when they were 38.

Five per cent of the cohort said they had started persistent cannabis use – defined as weekly sessions – before the age of 18.

These individuals were compared with the rest of the group both in terms of IQ and other possible interfering factors, as well evidence based on detailed interviews with friends and family.

"The people who used pot persistently as teens scored significantly worse on most of the tests," Dr Meier said. "Friends and relatives routinely interviewed as part of the study were more likely to report that the persistent cannabis users had attention and memory problems such as losing focus and forgetting to do tasks."

An average loss of eight IQ points out of an average score of 100 would mean a drop of IQ to 92, which would be enough to move someone from a group shared by 50 per cent of the population to a lower group shared by just 29 per cent of the population.

"Research has shown that IQ is a strong determinant of a person's access to a college education, their lifelong total income, their access to a good job, their performance on the job, their tendency to develop heart disease, Alzheimer's disease and even early death," Dr Meier said.

"Individuals who lose eight points in their teens may be disadvantaged, relative to their same-age peers, in most of the important aspects of life and for years to come."

Laurence Steinberg, a psychologist at Temple University in Philadelphia, said the study is the first to distinguish between cognitive problems that might result from using cannabis in adolescence, from those that existed prior to the cannabis use.

"The findings are pretty clear that it is not simply chronic use that causes deficits, but chronic use with adolescent onset," he said.

Start your day with The Independent, sign up for daily news emails
ebooks
ebooksA special investigation by Andy McSmith
  • Get to the point
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Recruitment Genius: Purchase Ledger Administrator

£5120 per annum: Recruitment Genius: You will be working for one of the countr...

Recruitment Genius: Engineering Surveyor

£20000 - £23000 per annum: Recruitment Genius: An opportunity has arisen for a...

Recruitment Genius: Customer Services Support

£9000 per annum: Recruitment Genius: You will be working for one of the countr...

Recruitment Genius: Junior Estimator

£17000 - £18000 per annum: Recruitment Genius: An opportunity has arisen for a...

Day In a Page

Not even the 'putrid throat' could stop the Ross Poldark swoon-fest'

Not even the 'putrid throat' could stop the Ross Poldark swoon-fest'

How a costume drama became a Sunday night staple
Miliband promises no stamp duty for first-time buyers as he pushes Tories on housing

Miliband promises no stamp duty for first-time buyers

Labour leader pushes Tories on housing
Aviation history is littered with grand failures - from the the Bristol Brabazon to Concorde - but what went wrong with the SuperJumbo?

Aviation history is littered with grand failures

But what went wrong with the SuperJumbo?
Fear of Putin, Islamists and immigration is giving rise to a new generation of Soviet-style 'iron curtains' right across Europe

Fortress Europe?

Fear of Putin, Islamists and immigration is giving rise to a new generation of 'iron curtains'
Never mind what you're wearing, it's what you're reclining on

Never mind what you're wearing

It's what you're reclining on that matters
General Election 2015: Chuka Umunna on the benefits of immigration, humility – and his leader Ed Miliband

Chuka Umunna: A virus of racism runs through Ukip

The shadow business secretary on the benefits of immigration, humility – and his leader Ed Miliband
Yemen crisis: This exotic war will soon become Europe's problem

Yemen's exotic war will soon affect Europe

Terrorism and boatloads of desperate migrants will be the outcome of the Saudi air campaign, says Patrick Cockburn
Marginal Streets project aims to document voters in the run-up to the General Election

Marginal Streets project documents voters

Independent photographers Joseph Fox and Orlando Gili are uploading two portraits of constituents to their website for each day of the campaign
Game of Thrones: Visit the real-life kingdom of Westeros to see where violent history ends and telly tourism begins

The real-life kingdom of Westeros

Is there something a little uncomfortable about Game of Thrones shooting in Northern Ireland?
How to survive a social-media mauling, by the tough women of Twitter

How to survive a Twitter mauling

Mary Beard, Caroline Criado-Perez, Louise Mensch, Bunny La Roche and Courtney Barrasford reveal how to trounce the trolls
Gallipoli centenary: At dawn, the young remember the young who perished in one of the First World War's bloodiest battles

At dawn, the young remember the young

A century ago, soldiers of the Empire – many no more than boys – spilt on to Gallipoli’s beaches. On this 100th Anzac Day, there are personal, poetic tributes to their sacrifice
Dissent is slowly building against the billions spent on presidential campaigns – even among politicians themselves

Follow the money as never before

Dissent is slowly building against the billions spent on presidential campaigns – even among politicians themselves, reports Rupert Cornwell
Samuel West interview: The actor and director on austerity, unionisation, and not mentioning his famous parents

Samuel West interview

The actor and director on austerity, unionisation, and not mentioning his famous parents
General Election 2015: Imagine if the leading political parties were fashion labels

Imagine if the leading political parties were fashion labels

Fashion editor, Alexander Fury, on what the leaders' appearances tell us about them
Phumzile Mlambo-Ngcuka: Home can be the unsafest place for women

Phumzile Mlambo-Ngcuka: Home can be the unsafest place for women

The architect of the HeForShe movement and head of UN Women on the world's failure to combat domestic violence