The persistent, dependent use of marijuana before age 18 has been shown to cause lasting harm to a persons 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 eight points between ages 13 and age 38.
Quitting marijuana did not appear to reverse the loss either, said lead researcher Madeline Meier, PhD, a post-doctoral researcher at Duke University.
The key variable in this finding is the age of onset for marijuana use and the brains development, Meier said. Study subjects who took up marijuana in adulthood 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, whose data came from the long-term 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% of people in the study group were considered marijuana-dependent or were using more than once a week before age 18. A dependent user was defined as one who keeps using despite significant health, social or family problems.
At age 38, all study participants were given a battery of psychological tests to assess memory, processing speed, reasoning and visual processing. The people who used marijuana persistently as teens scored significantly worse on most of the tests. Friends and relatives routinely interviewed as part of the study were more likely to report that the persistent cannabis users had attention and memory problems such as losing focus and forgetting to perform tasks.
The decline in IQ among persistent cannabis users could not be explained by alcohol or other drug use or by having less education, said Terrie Moffitt, PhD, a co-leader of the Dunedin study and a psychologist with dual appointments at Duke and the Institute of Psychiatry at Kings College London.
Although eight IQ points may not sound like a lot on a scale where 100 is the average, a drop 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 added: “Somebody who loses eight IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come.”
Laurence Steinberg, PhD, a Temple University psychologist who was not involved in the research, said the study is among the first to distinguish between cognitive problems the person might have had before taking up marijuana and those that apparently were caused by the drug. The findings are consistent with what has been found in animal studies, Steinberg added, but have been difficult to measure in humans.
Animal studies involving nicotine, alcohol and cocaine have shown that chronic exposures before the brain is fully developed can lead to more dependence and long-term changes in the brain. “This study points to adolescence as a time of heightened vulnerability,” Steinberg said. “The findings are pretty clear that it is not simply chronic use that causes deficits, but chronic use with adolescent onset.”
The study appeared Aug. 27 on the website of the Proceedings of the National Academy of Sciences. The study abstract is available at www.pnas.org/content/early/2012/08/22/1206820109.abstract.