Why it is SO dangerous for pre-teens to smoke pot: Study reveals weed's crippling damage on brains of under-17s

• Teens who smoked pot by age 14 performed worse by age 20 on cognitive tests
• They also had poor short-term memory and were likely to drop out of school
• But teens holding off until age 17 performed as well as those who didn't smoke

By Mary Kekatos

Marijuana is far more damaging to young teenage brains than we thought, a new study claims.

There has been a surge in teen cannabis smokers in the last few years, with a 2014 report saying about a third of teenagers try the drug before they reach 15.

But scientists warn new research shows that it is far better for their brains if they wait until after they turn 17.

Among other dangers, they found adolescents who smoked pot as early as 14 fared far in cognitive tests when they reached 20 - and dropped out of school at a much higher rate than non-smokers.

Teenagers who smoked marijuana as early as age 14 did worse on cognitive tests, decreased verbal abilities and were more likely to drop out of school, according to a new study

In the study, conducted by the Université de Montréal in Canada, a team looked at 294 teenage boys who completed a variety of cognitive tests at ages 13, 14 and 20.

Additionally, between the ages of 13 and 17 and again at 20, they filled out questionnaires.

Roughly half – 43 percent – reported smoking pot at some point throughout the course of testing, most of them only a few times a year.

At 20 years old, 51 percent said they still used the drug. In general, those who started early already had poor short-term memory and poor working memory. Conversely, the early users also had good verbal skills and vocabulary.

Lead author, Dr Natalie Castellanos-Ryan, an assistant professor at UdeM's School of Psychoeducation, suggested one possible explanation: 'It takes quite a lot of skills for a young adolescent to get hold of drugs. They're not easy-access.'

She and her team found smoking cannabis during adolescence was linked to later difficulties with verbal abilities and cognitive abilities of learning by trial-and-error.

The early adopters also tended to drop out of school sooner, which the researchers say helped explain the decrease in their verbal abilities.

'The results of this study suggest that the effects of cannabis use on verbal intelligence are explained not by neurotoxic effects on the brain, but rather by a possible social mechanism,' Dr Castellanos-Ryan said.

'Adolescents who use cannabis are less likely to attend school and graduate, which may then have an impact on the opportunities to further develop verbal intelligence.' 

 Adolescents who use cannabis are less likely to attend school and graduate, which may then have an impact on the opportunities to further develop verbal intelligence

 Dr Natalie Castellanos-Ryan

Besides filling out questionnaires about their use of drugs and alcohol over the previous year, the boys participated in a number of tests to measure their cognitive development.

They were given words and numbers to remember and repeat in various configurations and were asked to learn new associations between various image.

The boys also played a card game to gauge their response to winning or losing money, and, in a test of their vocabulary, had to name objects and describe similarities between words.

In general, those who reported smoking pot early in their young teens performed poorly on language tests and tests that required learning by trial-and-error.

However, the team found that if teenagers held off until age 17 before smoking their first joint, those adolescents performed as well as their peers who didn't smoke marijuana. 

Dr Castellanos-Ryan said she next wants to study whether the results can be replicated in other samples of adolescents and to see if cannabis use is associated with other problems, such as drug abuse, later in life.

But the main issue, she says, is to focus on delaying the onset of marijuana use as well as being 'realistic' with children.

She said: 'It is important to stick to the evidence we have and not exaggerate the negatives of cannabis.

'We can't tell children, "If you smoke cannabis you're going to damage your brain massively and ruin your life."

'We have to be realistic and say, "We are finding evidence that there are some negative effects related to cannabis use, especially if you start early, and so, if you can hold off as long as you can – at least until you're 17 – then it's less likely there'll be an impact on your brain".'