The study, which monitored over 1000 people from birth to the age of 38, found that cannabis has a more damaging effect on young brains than those of adults, researchers said.
The study, carried out on people from Dunedin, New Zealand, showed that those who had been habitual cannabis smokers in their youth experienced a drop of eight points on average in their IQ between the ages of 13 and 38.
The research also showed that those who started smoking cannabis after the age of 18 did not show the same decline in IQ.
The study was conducted by researchers from King’s College and Duke University, North Carolina, in the US.
Commenting on the results, Professor Terrie Moffitt of King’s College London’s Institute of Psychiatry told the Guardian newspaper: “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.”
Describing the significance of a drop in IQ of eight points, researcher Madeline Meier, from Duke, said it could mean the difference between being in the 50th and the 29th percentile.
“Somebody who loses eight IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come,” Meier told the Guardian.
One of the leaders of the research, Avshalom Caspi, of Duke and King’s, said: “The simple message is that substance use is not healthy for kids. That’s true for tobacco, alcohol, and apparently for cannabis.”
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