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Teenage stoners may be getting more than high when they smoke pot regularly. They may also be losing brain power permanently, according to a new study.

Researchers compared Intelligence Quotient (IQ) results for subjects aged 13 and then at 38 for more than 1000 New Zealanders, some of whom were regular cannabis users and some who were not.

The results were striking. A decline of about eight points for those who started smoking as teens and kept it up persistently in their 20s and 30s, said lead researcher Madeline Meier, a Duke University psychologist.

That's a big deal, Meier said.

On average, IQ should be stable as a person ages, she said.

For the people in the study who had never smoked marijuana, their IQ went up about a few tenths of a point.

"We know that IQ is a strong determinant of a person's access to a college education, lifelong total income, their access to a job, their performance on a job," Meier said.

"Somebody who loses eight IQ points in their teens and 20s may be disadvantaged compared to their same-age peers in most of the important aspects of life, and for years to come."

And the drop couldn't be traced to differences in education or by other substance abuse, including alcohol or other drugs, she said.

The ones who started smoking cannabis as adolescents and continued persistently also performed more poorly on tests of memory and ability to focus and think quickly, even when adjusted for each individual's natural abilities.

And those who quit or slowed their marijuana use within the year prior to testing at age 38 still showed the same deficits.

Yet for those persistent users who started smoking as adults, brain power didn't drop. This was a key distinction, Meier said.

"Adolescence is a particularly vulnerable period of brain development," Meier said.

Children who started smoking early might be disrupting the normal critical brain processes - permanently damaging their brains, she said.

The study did not gather data on exactly how much pot was used or how often the persistent users were smoking.

Those who showed deficits were those who researchers determined were "cannabis dependent" during periodic interviews from age 18 to age 38.

Further research could also help determine if staying off cannabis for more than a year meant "functioning could be recovered", Meier said.

"We didn't look into that, but it's definitely possible."

She said the results showed "cannabis use, marijuana use, in adolescence is not healthy".

"It's harmful," Meier said.