Lead researcher Madeline Meier, a post-doctoral researcher at Duke University, and a team of international colleagues said a long-range study cohort of more than 1,037 New Zealanders born in 1972-1973 found about 5 percent were considered marijuana-dependent, or were using more than once a week before age 18.
A dependent user is one who keeps using despite significant health, social or family problems, Meier said.
At age 38, all of the study participants were given a battery of psychological tests to assess memory, processing speed, reasoning and visual processing.
The study, published in the Proceedings of the National Academy of Sciences, found an average decline in IQ of 8 points when their age 13 and age 38 IQ tests were compared. Quitting pot did not appear to reverse the loss, the study said.
The decline in IQ among persistent marijuana users could not be explained by alcohol or other drug use or by having less education, Meier said.
"Somebody who loses 8 IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come," Meier said in a statement.