Duke University researchers determined in a new study on the effects of marijuana that teens who consume a lot of the herb before they turn 18 can suffer from impaired attention, memory and intelligence in the long run.
The Duke team led an international collaboration of scientists who investigated the issue. The group says that people who started using cannabis during adolescence, and continued to do so over the next few years, exhibit a decline in average IQ of around 8 points.
Madeline Meier, a postdoctoral researcher at the university, and the lead researcher on the new paper, says that no positive effect was seen on the brain even after consumers quit the drug. The changes therefore appear to be irreversible, PsychCentral
Interestingly, the research group did not find similar mental decline rates in people who started smoking pot after they turned 18. What this implies is that age – and perhaps the stage of brain development at which the habit starts – is a determining factor on what effects using pot will have.
Experts argue that the brain is still undergoing an active process of organizing and modeling itself, and this also makes it more vulnerable being damaged by drug use. After individuals reach early adulthood, using marijuana no longer has these effects on mental decline, the team says.
Data used in the new study came from the Dunedin Multidisciplinary Health and Development Study. The survey kept tabs on 1,037 people born in Dunedin, New Zealand, between 1972 and 1973. All participants were monitored until the age of 38.
“Marijuana is not harmless, particularly for adolescents,” Meier explains, adding that around 5 percent of people in the study were cataloged as marijuana-dependent. This meant that they used the drug commonly, even at great social and health risks.
“Somebody who loses 8 IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come,” Meier goes on to say. Previous studies, she continues, associated higher IQ levels with better income, better education, longer life spans and better overall health.
When people in the long-term survey were given a series of tests, at age 38, those who consumed marijuana as teens performed worse on memory, processing speed, visual processing and reasoning tasks than those who did not smoke the drug as teens, or began doing so beyond the age of 18.