Researchers from Britain and the United States found that persistent and dependent use of cannabis 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, a psychology and neuroscience professor at King's College London's Institute of Psychiatry, said the scope and length of the study, which involved more than 1,000 people followed up over 40 years, gave its findings added weight.

"It's such a special study that I'm fairly confident cannabis is safe for over-18 brains, but risky for under-18 brains," she said.

Before the age of 18, the brain is still being organized and remodeled to become more efficient and may be more vulnerable to damage from drugs, she added.

Moffitt worked with Madeleine Meier, a post-doctoral researcher at Duke University in the United States, to analyze data on 1,037 New Zealanders who took part in the study. About 96 percent of the original participants stuck with the study from 1972 to today, she said.

At age 38, all participants were given a battery of psychological tests to assess their memory, processing speed, reasoning and visual processing.

Those who had used pot persistently as teens scored significantly worse in most of the tests. Friends and relatives regularly interviewed as part of the study were more likely to report that the heavy cannabis users had attention and memory problems such as losing focus and forgetting to do tasks.

The researchers also 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.

"Study subjects who didn't take up pot until they were adults with fully-formed brains did not show similar mental declines," Moffitt said.

"MARIJUANA IS NOT HARMLESS"

She said the decline in IQ could not be explained by alcohol or other drug use or by having less education, and Meier said the key variable was the age people began to use pot.

Meier said the study's message was clear: "Marijuana is not harmless, particularly for adolescents."

While 8 IQ points may not sound like a lot on a scale where 100 is the mean, Meier said an IQ drop from 100 to 92 would mean dropping from being in the 50th percentile to being in the 29th.

Higher IQs also correlate with higher levels of education and income, better health and longer lives, she said. "Somebody who loses 8 IQ points as an adolescent may be disadvantaged ... for years to come," she added.

Robin Murray, a professor of psychiatric research at King's Institute of Psychiatry, who was not involved in this work, said the study was impressive and the findings should be taken "very seriously".

"It is of course part of folk-lore among young people that some heavy users of cannabis seem to gradually lose their abilities and end up achieving much less than one would have anticipated," he said in a statement. "This study provides one explanation as to why this might be the case."

Previous research on cannabis use has also pointed to potential long-term psychiatric effects.

A study published in March last year found that people who use it a lot in their youth dramatically increase their risk of psychotic symptoms, and that continued use of the drug can increase the risk of developing a psychotic disorder.

Meier pointed out that it was not possible to say from this latest study what a safer age for persistent pot use might be, or what kind of dosage level causes damage.

According to the 2011 United Nations Office for Drugs and Crime (UNODC) global drugs report, which used data from 2009, between 2.8 and 4.5 percent of the world's population aged 15 to 64 - or between 125 and 203 million people - had used cannabis at least once in the previous 12 months.

(Editing by Pravin Char)

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