How smoking cannabis can affect your intelligence: Using the drug once a week for six months can knock off two IQ points and could have ‘significant effects’ on teens’ verbal skills
- Researchers analysed data on more than 6,000 teenagers to track cannabis use
- Of the cohort in the study 808 used cannabis at least weekly for six months
- They found those regularly using cannabis as a teenager saw a two point IQ drop
- When it comes to verbal IQ – linked to reasoning – there is a three point IQ drop
Teens who smoke weed at least once a week for six months can lose up to two IQ points as they get older and find it harder to problem solve, a new study revealed.
The Royal College of Surgeons in Ireland (RCSI) studied 808 teens who used cannabis at least weekly for at least six months and 5,308 who did not use the drug.
They discovered that regular dope smokers suffer a decline of two IQ points over time compared to those who did not use cannabis during their teen years.
Further analysis showed that verbal IQ, linked to understanding concepts, abstract reasoning and memory, declines by three points among those who get high.
‘Loss of IQ points early in life could have significant effects on performance in school and college and later employment prospects,’ said senior author Mary Cannon.
The Royal College of Surgeons in Ireland (RCSI) studied 808 teens who used cannabis at least weekly for at least six months and 5,308 who did not use the drug (stock)
Cannabis is the most frequently used illicit substance worldwide, with young people the most common users, according to the team behind this new research.
In the UK alone 29.6 per cent of people aged 16-59 in the UK said they had used the drug at least once in their lifetime – up from 23.6 per cent in 2002.
MRI scans taken as part of the study also revealed that those who smoked weed – even minimal incidental use while a teenager – showed evidence of reduced grey matter in the brain.
Study first author Dr Emmet Power, Clinical Research Fellow at RCSI, said there was a clear association between frequent or dependent cannabis use and IQ change.
‘This corresponds to a 1.98-point decline in IQ,’ Dr Power said of the results.
‘We extracted verbal IQ change effect sizes from four available studies This corresponds to a decline of 2.94 verbal IQ points.
They discovered that regular dope smokers suffer a decline of two IQ points over time compared to those who did not use cannabis during their teen years (stock)
‘We found that young people who use cannabis frequently or dependently by age 18 have declined in IQ at follow up and this may be due to a decline in verbal IQ.
‘All studies showed point estimates of IQ decline,’ Power added.
Researchers used data from a number of studies, which limited the available detail as all but one only followed people up to the age of 18 the other one went until 38.
The two point decline in IQ is ‘not to be clinically significant’ and is unlikely to completely explain a range of psychosocial problems linked to cannabis, he said.
Dr Power added: ‘Cannabis use during youth is of great concern as the developing brain may be particularly susceptible to harm during this period.
‘The findings help us to further understand this important public health issue.’
Professor Mary Cannon, senior author and professor of psychiatric epidemiology, said previous studies have shown people who use cannabis regularly when young have worse outcomes in life than non-cannabis users.
Adding that they ‘are at increased risk for serious mental illnesses like schizophrenia.’
She said this small drop in IQ points could lead to significant effects on performance in school, college and life generally which could then result in limited employment prosecutes.
The findings were published in Psychological Medicine.