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A leading campaigner for cannabis law reform says "Dunedin Study" research linking heavy use of marijuana before the age of 18 to loss of IQ backs his view that cannabis should be legalised.
The study, which has been picked up by media outlets all over the world, found that those dependent on cannabis in adolescence suffered an average decline of eight IQ points from when they were tested at age 13 and again at age 38.
Those who did not take up pot until they were adults "with fully formed brains" did not show similar mental declines.
Norml vice-president and Dunedin resident Abe Gray said the findings strengthened the case for legalising cannabis and putting in an age restriction preventing people under the age of 18 from buying it.
Legalising cannabis and introducing an age restriction would result in fewer young people smoking cannabis, as was seen in the Netherlands, which had the "lowest rate of teen cannabis use of any Western country", Mr Gray, who is a teaching fellow at the University of Otago, said.
He had faith in the results of the study, but said the effects of heavy cannabis use before the age of 18 differed depending on the individual.
As someone who smoked cannabis heavily before the age of 18 he did not believe his IQ had been affected.
"It doesn't appear that way, although who's to say I wouldn't have been prime minister by now if I hadn't smoked it."
Meanwhile, the research has been picked up by hundreds of media outlets across the world, including The Guardian newspaper and the BBC in Britain and Fox News and USA Today in the United States.
The research used data from the Dunedin Multidisciplinary Health and Development Study, also known as the "Dunedin Study", which has followed a group of 1037 children born in 1972-73 in Dunedin from birth.