The study found links between cannabis use and brain impairment only in the areas of verbal IQ and specific cognitive abilities related to frontal parts of the brain, particularly those that require learning by trial-and-error.
“We found that adolescents who started using cannabis at 17 or older performed equally well as adolescents who did not use cannabis,” said lead author Natalie Castellanos-Ryan, from University of Montreal (UdeM) in Canada.
“Overall, these results suggest that, in addition to academic failure, fundamental life skills necessary for problem-solving and daily adaptation may be affected by early cannabis exposure,” said the researchers.
As many as 294 teenagers completed a variety of cognitive tests at ages 13, 14 and 20 and filled out a questionnaire once a year from ages 13 to 17 and again at 20, between 1991 and 1998.
Roughly half -43 per cent -reported smoking pot at some point during that time, most of them only a few times a year.
At 20 years of age, 51 per cent said they still used the drug. In general, those who started early already had poor short-term memory and poor working memory (the ability to store information such as a phone number long enough to use it, or follow an instructionSHORTLY after it was given).
Conversely, the early users also had good verbal skills and vocabulary.
“It takes quite a lot of skills for a young adolescent to get hold of drugs; they’re not easy-access,” said Castellanos-Ryan.
The team found smoking cannabis during adolescence was only linked to later difficulties with verbal abilities and cognitive abilities of learning by trial-and-error and those abilities declined faster in teens who started smoking early than teens who started smoking later.
The early adopters also tended to drop out of school sooner, which helped explain the decrease in their verbal abilities.
The study was published in the journal Development and Psychopathology.