For years, regular marijuana users have dismissed cautionary warnings that the habit killed brain cells, believing it was some kind of urban myth. Well, now scientists have produced evidence to back up the myth: a study of more than 1,000 people in New Zealand who were followed over two decades found that those who started habitually smoking marijuana before age 18 eventually showed an 8-point drop in IQ.
The study, published today in the Proceedings of the National Academy of Sciences, will give parents hefty ammunition in those anti-drug talks with their kids.
Because the average IQ is 100 points, a drop of just eight points in IQ would mean falling from the 50th percentile to the 29th percentile in terms of intelligence—in other words, a drop from average to way below average.
The researchers started with a baseline intelligence for all study subjects, testing them when they were under 13 and hadn’t yet started smoking and then assessing them at five different points in their lives—ages 18, 21, 26, 32, and 38. They again tested IQ at age 38. The researchers assessed marijuana use and tested intelligence. he authors also controlled for alcohol use, other drug use and education level.
According to the study, the eight-point drop was found in subjects who started smoking in adolescence and persisted in “habitual smoking”—using it at least four days per week—in three or more of the five testing points.
People who started smoking in adolescence but used marijuana less persistently still had a hit to their IQ’s, but it was less pronounced than the group that used it early and persistently.
Those who never used marijuana actually got smarter by nearly one IQ point on average.
Madeline Meier, lead researcher and a post-doctoral associate at Duke University, said that persistent use of marijuana in adolescence appeared to blunt intelligence, attention and memory. More persistent marijuana use was associated with greater cognitive decline.
“Collectively, these findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects,” Meier writes in the study.
Alarmingly, subjects who stopped using marijuana for a year still showed the persistence of adverse effects and some neurological deficits. But those who didn’t start smoking until after adolescence showed no adverse effects on intelligence.
Experts in child development said the reasons adolescents may be more susceptible to the harmful effects of marijuana may have to do with a substance called myelin. Myelin can be thought of as a kind of insulation for nerve cells in the brain that also helps speed brain signals along — and in adolescent brains, the protective coating it forms is not yet complete.
“Frontal lobe myelination is not fully completed until age 25 years or so, and the pre-myelinated brain is more susceptible to damage from neurotoxins,” says Dr. Richard Wahl, director of adolescent Medicine at the University of Arizona. “Cannabis, most likely, is a neurotoxin in high and continuous doses.”
According to statistics released in June by the U.S. Centers for Disease Control and Prevention, American teenagers today are more likely to be using marijuana than tobacco products. Of particular worry is the attitude that marijuana is one of the more harmless drugs.
“Increasing efforts should be directed toward delaying the onset of cannabis use by young people,” writes Meier, “particularly given the recent trend of younger ages of cannabis-use initiation in the United States and evidence that fewer adolescents believe that cannabis use is associated with serious health risk.”