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Teens who become addicted to cannabis before age 18 may suffer permanent damage to their intelligence, memory and attention, according to results of an extensive long-term study published.

Researchers from Britain and the United States found that continued use and cannabis-dependent manner by 18 years could have a neurotoxic effect, but after that age seem to be less harmful to the brain.

Terrie Moffitt, a psychologist and professor of neuroscience at the Institute of Psychiatry at King’s College London, said the scope and extent of the study, which involved tracking 1,000 people over 40 years, gives the results other weight.

“It’s a very special study I have enough confidence that cannabis is safe for brains over 18 years, but risky for children under 18,” he said.

Before 18, the brain is still organizing and remodeling to be more efficient and perhaps more vulnerable to damage from drugs, he added.

Meier worked with Madeleine Moffit, a Ph.D. researcher at Duke University in the U.S., to analyze data of 1,037 New Zealanders in the study.

Approximately 96 percent of the original members remained in the study from 1972 to the present. At 38 years, all participants underwent a series of psychological analysis to assess their memory, processing speed, reasoning and visual processing.

Those who had used marijuana as teenagers consistently reported significantly worse in most analyzes.

Friends and family frequently interviewed as part of the study reported that cannabis smokers had problems with attention and memory.

The researchers also found that people who started using cannabis in adolescence and continued to smoke for years showed an average decrease in test scores of intelligence quotient (IQ) of 8 points for the ages between 13 and 38.

“Study subjects who started using cannabis when they were adults with fully formed brains showed no similar mental declines,” said Moffitt.

“Marijuana is not harmless”

Moffit said the decline in IQ could not be explained by the use of alcohol or other drugs or have less education, and Meier said the key variable was the age when people had started using marijuana.

Meier said the study’s message was clear: “Marijuana is not harmless, especially for adolescents.” While 8 IQ points may not sound like much on a scale where 100 is average, Meier said a decline in IQ of 100-92 would fall from the 50th percentile to the 29th.

Higher IQs also correlate with higher levels of education and income, better health and longer life, said Meier. “Someone who loses 8 IQ points in adolescence may be disadvantaged (…) for the future,” he said.

Robin Murray, professor of psychiatric research at the Institute of Psychiatry, Kings, who was not involved in the study, said the work was admirable and the results should be taken “very seriously.”

“Of course it’s part of folklore among young people that some heavy cannabis consumers seem gradually lose their abilities and end up making much less than you had anticipated,” said in a statement.

“This study provides an explanation of why this would happen,” he added. Previous research on the use of cannabis have also pointed to the potential long-term psychiatric effects.

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

Meier said it was not possible to establish the latter study what would be the safest age for continuous use of marijuana, or how much damage it can cause.

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