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Teenage marijuana use causes mental deficit

A nearly four decade study led by Duke researchers holds harrowing implications for adolescent marijuana users.

The study followed more than 1,000 people from the time they were born and found that individuals who used marijuana regularly in early adolescence suffered on average an eight point IQ decline by the time they reached age 38. Not only is this drop in IQ irreversible, but it is enough to knock a person of average intelligence from the 50th percentile to the 29th percentile.

“Those who started to use cannabis a lot between 16 and 18 were the kids who experienced a lot of the IQ decline,” Avshalom Caspi, an Edward M. Arnett professor of psychology and neuroscience and co-author of the study, said. “Even among people who had quit…if they started very early and used cannabis for many years, their neurological test scores didn’t seem to rebound.”

An IQ drop of eight points has a variety of long-term ramifications for individuals. Not only does IQ influence college admission and job placement, but it can even have an effect on mortality, said Madeline Meier, a post-doctoral researcher and lead author of the study.

The findings only pertain to individuals who were early onset and dependent users. Those who started using marijuana before turning 18 were categorized as early-onset users, and those who continued using even when there were significant health or social consequences were labeled dependent users.

As a result, the findings only pertain to a small segment of the population. Only five percent of the individuals studied were considered early-onset and dependent users and suffered from a significant drop in IQ.

“Some people may look at these findings and say, ‘Well, we knew that all along, we knew stoners and what they were like in high school,’” Caspi said. “But the findings are more nuanced than that because they suggest we must really pay attention to sensitive periods in development where substance use may have more harmful effects than it does in other points in development.”

Although people generally accept that cigarettes are not good for you, many will maintain that marijuana is not harmful—an idea that has been propelled by the medical marijuana movement. The findings reverse any perception that marijuana is completely harmless, Meier said.

But the research could also be used to support marijuana use, Caspi said.

“A lot of people have believed for a long time that cannabis really isn’t harmful and is a wonderful recreation drug,” he said. “There is evidence for that [argument] as well, since many people in our research have smoked cannabis on and off in their lives without any adverse effects on their function.”

Regardless of how the study is ultimately interpreted, the findings highlight that a heavy reliance on drugs during adolescence can haunt an individual far into the future.

“Certain maturational changes occur in the brain from age 13 to up until the early twenties and adolescents who use cannabis are interrupting these critical brain changes,” she said. “To anybody who is an adolescent whose brain is still developing, which can be up to the early twenties, marijuana is not harmless.”

Thomas Szigethy, associate dean and director of the Duke Student Wellness Center, wrote in an email Wednesday that marijuana is the second most heavily used substance on campus, after alcohol. Although some students think pot is less dangerous than alcohol, it can still adversely affect their academic and social lives.

“Many more people smoking pot lose motivation,” Szigethy said. “Ultimately, people need to assess why they are using a substance—what are they avoiding [and] why can they not accomplish the same positive feeling about life without using [this] substance?”


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