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

Researchers compared Intelligence Quotient (IQ) results for subjects aged 13 and then at 38 for more than 1,000 New Zealanders, some of whom were regular cannabis-users and some who were not.

The results were striking. A decline of around eight points for those who started smoking up 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 explained in a telephone interview with AFP. "On average, IQ should be stable" as a person ages, she said.

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

"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 twenties 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 is a key distinction, Meier said.

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

Those kids who smoke up early and often "might be disrupting the normal critical brain processes," permanently damaging their brains, she explained.

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."

But bottom line, she said, the results show "cannabis use, marijuana use, in adolescence is not healthy. It's harmful."

The study was published Monday in the Proceedings of the National Academy of Sciences.