woman blows smoke from a marijuana cigarette during a pro-marijuana rally in Buenos Aires May 5, 2012.
Credits: REUTERS/Enrique Marcarian
A team of international researchers found the IQ of men and women who smoked weed as teens and for years afterward declined an average of eight points compared to non-drug users. Quitting didn't reverse the results.
But those who started smoking pot after the age of 18 didn't have a mental decline, the study said.
Before age 18, the brain is still being organized and remodelled to become more efficient and may be more vulnerable to damage from drugs, said lead researcher Madeline Meier, a post-doctoral researcher at Duke University in the U.K.
Researchers studied a cohort of more than 1,000 people from New Zealand. They were given psychological tests at ages 13 and 38, and researchers compared the results. The marijuana-dependent group, who smoked pot more than once a week before they were 18, scored "significantly worse on most of the tests," the study found.
The IQ drop couldn't be explained by booze, other drugs or having less education, the authors said.
"The simple message is that substance use is not healthy for kids," co-author Avshalom Caspi said in a statement. "That's true for tobacco, alcohol, and apparently for cannabis."
The study was published Monday in the Proceedings of the National Academy of Sciences journal online.