Illegal drug use by young people has been decreasing since the mid 1990’s, but the rate of decline for marijuana use, specifically, has been very slow.
In the past, research studies have demonstrated the short term effects of cannabis but examining the long term effects were at a disadvantage as the longitudinal studies lasted 10 years or less.
A new study, published in August 2012 in the Proceedings of the National Academy of Sciences, followed 1,000 people in New Zealand for 25 years.
The international team of researchers found that people who started using marijuana below the age of 18, while their brains were still developing, demonstrated a drop in IQ points.
The new study doesn’t prove that prolonged marijuana use impairs intellectual functioning, but it does provide very strong evidence of a cause-and-effect relationship.
Prior to this study, there was a 2002 study published in the Canadian Medical Association Journal that concluded that occasional or light cannabis use did not have a long term negative effect on global intelligence. They also noted that current use had a negative impact on global intelligence only when the subject smoked 5 or more joints per week.
They examined the subjects at ages 9 to 12 years (before they used marijuana) and again at ages 17 to 20. It found that subjects who had never used, current light users and former users showed modest IQ gains ranging from 2.6 to 5.8 points.
In contrast, the only group to show an actual decrease in IQ points (4.1 points) was the group of subjects who were considered heavy users.
In 2008, there was a small study (sample size of only 31 subjects) that indicated long-term, heavy marijuana use (over 5 joints daily for more than 10 years) was associated with structural abnormalities in the hippocampus and amygdala areas of the brain. Both of these areas tended to be smaller in the heavy, long term cannabis users.
The hippocampus regulates emotions and memory while the amygdala is involved with fear and aggression, although both of these areas of the brain are closely related to learning and memory processes.
The study concluded that “heavy daily cannabis use across protracted periods exerts harmful effects on brain tissue and mental health”.
What did the new study find?
In the new study, those who began smoking marijuana before the age of 18 and “habitually smoked” (at least 4 days per week) as an adult showed an average drop of 8 points in IQ between when they were first tested at around 13 and when they were re-tested at age 38.
Although 8 points may not seem like much, that means a person who was at the 50th percentile would fall to the 29th percentile which is a considerable decline.
Furthermore, the heavier the use, the more cognitive decline was observed. People who started using cannabis as a teenager but used it less persistently also showed a decline in IQ points but it was less pronounced.
Those subjects who never engaged in marijuana use, on average, gained a point.
Madeleine Meier, the lead researcher, reported that persistent use of marijuana in adolescence appeared to blunt intelligence, attention and memory. She wrote, “Collectively, these findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects.”
The age of onset of smoking marijuana appears to be the significant variable. Users who began in adolescence and persisted into adulthood demonstrated IQ point declines, and quitting cannabis did not appear to reverse the loss.
Interestingly, those who started after the age of 18 did not show declines in memory, attention or IQ.
The study found that habitual pot smokers showed deficits in memory, concentration and general cognitive functioning in relation to their peers. These problems were even more pronounced when the habit was initiated during the teen years.
Individuals who smoked heavily in adolescence had consistently lower IQ’s at age 38, even if they had quit or cut back in the previous year. By contrast, the IQ of those who began cannabis use after age 18 was linked to how much pot they had smoked recently.
Why are Adolescent Brains More Vulnerable?
There are different theories as to why the adolescent brain is more susceptible to the effects of marijuana. The brain is still developing in adolescence; neurons are growing and changing, and synapses are forming.
Some theorize that the adolescent brain, because it is still growing and changing, is more vulnerable to environmental influences and that cannabis acts as a neurotoxic assault.
As previously mentioned, some believe that structural abnormalities are more likely to develop since the adolescent brain is still in a state of change.
Yet another theory involves cannabis and the formation of myelin; myelin acts as insulation for the brain’s nerve cells. It is not fully developed until around age 25. Therefore, since the myelination of the brain is not complete it leaves the brain more susceptible to damage from neurotoxins.
Of concern is the permanency of the cannabis effects among those in the study who began smoking marijuana as adolescents.
Even after the research participants stopped using marijuana for a year, its adverse effects persisted and some neurological deficits remained.
Teenagers who engage in frequent or heavy marijuana use may be setting themselves up for declines in cognitive function that persist into adulthood.