Delay delay delay. That’s the general catchcry of health specialists when commenting on teenagers’ engaging in risky behaviour involving alcohol, drugs and sex. And now there is even further evidence why smoking marijuana during adolescence is a bad idea.
A new long-range study of 1,000 New Zealanders found that participants who started using cannabis in adolescence and smoked it more than once a week, and continued this practice into adulthood, showed an average decline in IQ of 8 points between the ages of 13 and 38.
Study subjects who didn’t take up pot smoking until they were adults did not experience the same decline in IQ.
What does a loss of 8 IQ points mean?
A loss of just 8 IQ points can mean long-term disadvantage. The average IQ is 100. Dropping to 92 IQ points means going from being in the 50th percentile to being in the 29th.
“Higher IQ correlates with higher education and income, better health and a longer life,” said lead researcher Madeline Meier. “Somebody who loses 8 IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come.”
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At age 38, the study participants were tested 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. Friends and relatives interviewed for the study were more likely to report that the persistent cannabis users had attention and memory problems such as losing focus and forgetting to do tasks.
The researchers found that the subjects’ IQ did not improve after they quit smoking marijuana as an adult.
Adolescent brain more vulnerable
The study has shown that taking up pot-smoking in adolescence when the brain is still developing is particularly risky.
“Study subjects who didn’t take up pot until they were adults with fully-formed brains did not show similar mental declines,” said Dr Meier. “Before age 18, however, the brain is still being organised and remodeled to become more efficient, and may be more vulnerable to damage from drugs.”
“Marijuana is not harmless, particularly for adolescents,” said Meier.
Laurence Steinberg, a Temple University psychologist who was not involved in the research, said this study is among the first to distinguish between cognitive problems the person might have had before taking up marijuana, and those that were apparently caused by the drug.
“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 was published in the Proceedings of the National Academy of Sciences.