The persistent use of marijuana before the age of 18 has been shown to cause lasting harm to a person’s intelligence, attention and memory, according to an international research team.
A new study shows that people who started using cannabis in adolescence and used it for years afterward showed an average decline in IQ of 8 points.
Quitting pot did not appear to reverse the loss, added lead researcher Madeline Meier, a postdoctoral researcher at Duke University.
Age is the key variable, according to the researcher, who notes that study subjects who didn’t start smoking pot until after the age of 18 did not show similar mental declines.
Before age 18, however, the brain is still being organized and remodeled to become more efficient, she said, and may be more vulnerable to damage from drugs.
“Marijuana is not harmless, particularly for adolescents,” said Meier, who produced this finding from the Dunedin Multidisciplinary Health and Development Study, which followed a group of 1,037 children born in 1972-73 in Dunedin, New Zealand, from birth to age 38.
About 5 percent of the study group 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 explained.
At age 38, all of the participants were given psychological tests to assess memory, processing speed, reasoning and visual processing.
The people who used pot persistently as teens scored significantly worse on most of the tests, according to the researcher.
Friends and relatives routinely interviewed as part of the study were more likely to report that the persistent pot smokers had attention and memory problems, such as losing focus and forgetting to do tasks.
While an 8 point decline in IQ may not sound like a lot on a scale where 100 is the mean, a loss from an IQ of 100 to 92 represents a drop from being in the 50th percentile to being in the 29th, Meier said.
Higher IQ correlates with higher education and income, better health and a longer life, she 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.
Source: Duke University