Cannabis is not harmless, particularly for adolescents, the researchers say Link to this video
Adolescents who are regular users of cannabis are at risk of permanent damage to their intelligence, attention span and memory, according to the results of research covering nearly four decades.
The long-term study which followed a group of over 1,000 people from birth to the age of 38 has produced the first convincing evidence, say scientists, that cannabis has a different and more damaging effect on young brains than on those of adults.
Around 5% of the group used cannabis at least once a week in adolescence or were considered dependent on it. Between the age of 13 and 38, when all members of the group were given a range of psychological tests, the IQ of those who had been habitual cannabis users in their youth had dropped by eight points on average.
Giving up cannabis made little difference – what mattered was the age at which young people began to use it. Those who started after the age of 18 did not have the same IQ decline.
“This work took an amazing scientific effort,” said Professor Terrie Moffitt of King’s College London’s Institute of Psychiatry, one of the authors.
“We followed almost 1,000 participants, we tested their mental abilities as kids before they ever tried cannabis, and we tested them again 25 years later after some participants became chronic users.
“Participants were frank about their substance abuse habits because they trust our confidentiality guarantee, and 96% of the original participants stuck with the study from 1972 to today.
“It’s such a special study that I’m fairly confident that cannabis is safe for over-18 brains, but risky for under-18 brains.”
The research, on people in Dunedin, New Zealand, was carried out by researchers from King’s College and Duke University, North Carolina in the United States and published online by PNAS (Proceedings of the National Academy of Sciences).
“Marijuana is not harmless, particularly for adolescents,” said Madeline Meier from Duke, one of the researchers. While eight IQ points on a scale where the mean is 100 may not sound a lot, she said, a drop from 100 to 92 represents a move from the 50th to the 29th percentile. Higher IQs correlate with higher education and income, better health and a longer life. …