The persistent, dependent use of marijuana before age 18 has been shown to cause lasting harm to a person's intelligence, attention and memory, according to an international research team.
Among a long-range study cohort of more than 1,000 New Zealanders, individuals who started using cannabis in adolescence and used it for years afterward showed 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 either, said lead researcher Madeline Meier, a post-doctoral researcher at Duke University. The results appear online Aug. 27 in PNAS.
The key variable in this is the age of onset for marijuana use and the brain's development, Meier said. Study subjects who didn't take up pot until they were adults with fully-formed brains 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 long term Dunedin Multidisciplinary Health and Development Study.
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.
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 people who used pot persistently as teens scored significantly worse on most of the tests.