Regular use of marijuana by adolescents causes lasting harm to a person’s intelligence, attention and memory, according to an international research team.
In a long range study of over 1,000 New Zealanders, people who started using cannabis regularly as adolescents showed an average decline in their intelligence quotient (IQ) of 8 points. Quitting marijuana did not appear to reverse the loss. The study is being published in the Proceedings of the National Academy of Sciences.
“Marijuana is not harmless, particularly for adolescents,” said lead researcher Madeline Meier, a post-doctoral researcher at Duke University, who believes the key variable is the age at which marijuana use begins. Before age 18, she says the brain is still developing and may be more vulnerable to damage from drugs.
The study followed a group of 1,037 children born in 1972-73 in Dunedin, New Zealand from birth to age 38. All of the participants in the Dunedin Multidisciplinary Health and Development Study were given a battery of psychological tests to assess their memory, processing speed, reasoning and visual processing. About 5 percent of the study group were using marijuana more than once a week before age 18.
Those who used pot regularly as teens scored significantly worse on most of the tests. Their friends and relatives were also more likely to report that the persistent cannabis users had attention and memory problems. Study participants who did not start using marijuana until they were adults did not show mental declines.
Researchers say the decline in IQ among cannabis users could not be explained by other factors such as less education.
The loss of 8 IQ points can be significant for someone of average intelligence. Higher IQ correlates with higher education and income, better health and a longer life. “Somebody who loses 8 IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come,” said Meier.
Previous research on animals has shown that chronic exposure to nicotine, alcohol and cocaine before the brain is fully developed can lead to addiction and long-term changes in the brain.
“This study points to adolescence as a time of heightened vulnerability,” said Laurence Steinberg, a Temple University psychologist who was not involved in the research. “The findings are pretty clear that it is not simply chronic use that causes deficits, but chronic use with adolescent onset.”
What isn’t clear from the New Zealand study is what a safe age for persistent use might be, or what dosage level causes the damage. According to a recent study, the number of American teenagers regularly using marijuana has risen sharply since 2008. Over one in four teens have used marijuana in the past month and 1.5 million teens smoke marijuana heavily.
“The simple message is that substance use is not healthy for kids,” said Avshalom Caspi, a psychologist at Duke University and the Institute of Psychiatry at King’s College London. “That’s true for tobacco, alcohol, and apparently for cannabis.”
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