Cannabis inflicts long-term damage on teenage brains: Those who use the drug as teens struggle with reasoning, memory, and inhibitions in later life

Teenagers who use cannabis are inflicting long-term damage on their brains, a new study has warned.

Researchers tracked nearly 4,000 teenagers over fours years and found clear evidence of marijuana use being linked to struggles with reasoning, memory and inhibitions later in life.

Previous studies have shown that cannabis misuse has been linked to impairments in learning, attention, and decision-making, as well as lower academic performance. 

But the team, led by the University of Montreal in Canada, says its findings are the first to show the causal and lasting effects of teen pot use on cognitive development. 

For the study, the team followed more than 3,800 Canadian adolescents between seventh and 10th grade for a period of four years.

Once a year, the participants rated how much they used cannabis on a zero to five scale - zero meaning 'never' and five being 'every day'.

For alcohol specifically, the teens were asked to provide the typical number of drinks they have in a day.

Researchers also looked at year-over-year changes in four cognitive areas: recall memory, working memory, perceptual reasoning and inhibitions.

Tasks included being able to find a phone among a group of images, learning a pattern and being able to reproduce it 30 minutes later, and completing a sequence of puzzles with increasing difficulty.

The number of students who reported never using cannabis fell from 95.4 percent in the first year of the study to 71 percent in the last year. 

Meanwhile, teens who reported using marijuana every day increased five-fold from 0.37 percent in year one to two percent in year four. 

Using an advanced analytical model, the study found that teens who used cannabis more frequently performed the worst on the cognitive domain tests.  

Researchers also found that cannabis use in any given year was linked to impaired inhibitory control and working memory one year later.

'We were surprised that the effects of cannabis were more pronounced than the effects of alcohol,' senior author and investigator Dr Patricia Conrod, of the department of psychiatry at the University of Montreal, told Daily Mail Online.

'And we were surprised of the lasting effects. Even if a young person reduced their use, you could still see effects from the previous year. It was more than we expected.'

There is also a concern of teens vaping cannabis after a Centers for Disease Control and Prevention (CDC) report revealed last month e-cigarettes were being used to concel marijuana.

The report found one-third of high school students and a quarter of middle school students that had used e-cigarettes for cannabis in 2016.

A 2017 study also from the University of Montreal found that teens who smoked pot as early as 14 performed worse by age 20 on cognitive tests and dropped out of school at a much higher rate than non-smokers.

Another study from Concordia University, also located in Montreal, found that adolescents who smoked pot at age 15 or younger suffered from cognitive development and even respiratory diseases.

Dr Conrod said that there are many questions that she and her team would like to answer in future research.  

'In the context of recreational cannabis use in adults, when the brain not undergoing maturation, we want to see if they are just as vulnerable to the effects of cannabis,' she said.

'We also want to see if there's a difference in sensitivity between males and females and what brain mechanisms are underlying this effect.'