Story highlights Teens who smoke pot heavily lost an average 8 IQ points before age 38

Brainpower deficits are more pronounced among those who began smoking as teens

The new study provides evidence of a cause-and-effect relationship

(Health.com) Teenagers who frequently smoke marijuana may be setting themselves up for declines in intelligence and mental function that persist well into adulthood, new research suggests.

In a decades-long study of more than 1,000 New Zealanders, researchers found that adolescents who used marijuana at least four days per week lost an average of eight IQ points between the ages of 13 and 38 -- a pattern not seen among people who began smoking heavily only in adulthood.

Heavy pot smokers tended to show deficits in memory, concentration, and overall brainpower in relation to their peers, but these problems were more pronounced -- and seemingly more lasting -- among those who picked up the habit as teens, the study found.

Individuals who smoked heavily in adolescence had consistently lower IQs at age 38, even if they'd cut back in the previous year. By contrast, the IQ of the relative latecomers to marijuana was more closely linked with how much pot they'd smoked recently.

"The effect of persistent cannabis use on intellectual functioning is really confined to adolescents, (which) suggests that adolescents, in particular, are vulnerable to the effect of cannabis," says lead author Madeline H. Meier, a psychologist and postdoctoral researcher at Duke University in Durham, North Carolina.

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