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Pro-cannabis campaigners said that a major study into the effect of long term and persistent cannabis use strengthen the argument in favour of legalisation.
Abe Gray, of the National Organisation for the Reform of Marijuana Laws in New Zealand said he hoped the message "would not get turned around".
Professor Robin Murray from the Institute of Psychiatry at King's College London has called a major study into the effect of long term and persistent cannabis use "very impressive".
Professor Murray said that if the same results were found in other research, public education campaigns should be launched:
New research in to cannabis use has found that persistent and dependent use of the drug before the age of 18 may have a so-called neurotoxic effect, but heavy pot use after 18 appears to be less damaging to the brain. Terrie Moffitt from King's College London said:
Researchers found that people who started using cannabis in adolescence and continued for years afterwards showed an average decline in Intelligence Quotient (IQ) test scores of 8 points between the age of 13 and 38.
Madeline H Meier lead a team of researchers who worked with 1,037 people from Dunedin, New Zealand born between 1972/1973. She said an average decline of 8 points may not seem like a lot, but was significant as those with a higher IQ are more like to earn more, have better health and a longer life:
Teenagers who regularly smoke cannabis risk permanently damaging their intelligence, attention span and memory, according to one of the biggest reports into the impact of the long term impact of the drug.
Researchers from King's College London and Duke University in the United States followed the development of over 1,000 people from birth to the age of 38 and found that cannabis has a different and more damaging effect on young brains that on those of adults.