The study included information on more than 1,000 people born in New Zealand in 1972-1973. Participants took IQ and other mental functioning tests at age 13 — before any had started smoking marijuana — and then again at age 38.
Every few years, starting at age 18, participants were also asked about their use of marijuana and assessed for marijuana dependence. Marijuana dependence is defined as someone who feels they need to smoke more and more marijuana to get the same effect, who has tried to quit but can’t or who keeps using even though the habit is causing them problems, such as with their health, family, work or school.
About 5% reported using marijuana more than once a week before age 18 or were considered marijuana-dependent at one or more points during the study.
Those who started smoking marijuana heavily as teens — meaning at least once or week — or were diagnosed with marijuana dependence before age 18 and who continued to smoke into adulthood showed an average 8-point drop in their IQs by age 38.
People who started using marijuana heavily later — not as teenagers — also experienced an IQ drop, but it wasn’t as severe, the investigators found.
“The findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects,” said study author Madeline Meier, a postdoctoral research associate in psychology and neurology at Duke University.
For those who had started as teens, quitting or cutting back on marijuana later didn’t seem to help much, according to the study. Even if they stopped, intellectual functioning never came back to the previous levels.
Though pot has a reputation among many teens for being benign, Meier urged adolescents and their parents to take the findings seriously.
“As an adolescent, your brain hasn’t fully developed. It’s undergoing some critical developmental changes. This research suggests that because of that you are vulnerable to the effects of cannabis on your brain. If you start using as an adolescent and you keep using it, you are going to lose some of your mental abilities,” Meier said.
The study is published in this week’s online issue of the Proceedings of the National Academy of Sciences.
Among participants in the study, those who never smoked marijuana saw no drop, and even a slight rise, in their IQs from age 13 to 38.
So what’s the significance of an 8-point drop? A lot, the researchers said.
Average IQ is about 100, putting a person in the 50th percentile for intelligence compared to same-age peers. An 8-point drop from, say, 100 would put a person into the 29th percentile for intelligence, significantly below average, Meier said.
Researchers also took into account alcohol and other drug use, as well as reduced years of education, among persistent marijuana users, which could have also affected their IQs. But the IQ decline could not be explained by those other factors.
The study also found that friends and relatives reported noticing more attention and memory problems in everyday life among persistent cannabis users, such as losing focus when they should be paying attention, forgetting to do errands, return calls or pay bills, Meier said.
Harris Stratyner, vice president of Caron Treatment Centers’ New York Clinical Regional Services, said prior research has suggested that marijuana may impact development of multiple brain regions, including the prefrontal cortex, which is involved with high-level reasoning, and the hippocampus, which is involved with memory.
Teens who are using marijuana believing that it’s not harmful are fooling themselves, he added.
“Marijuana is very dangerous drug to the brain, particularly in adolescence, and it’s also a carcinogen, so it’s not healthy for adults, either,” Stratyner said. “It’s much more dangerous than we’ve ever given it credit for.”
In recent years, marijuana plants have also been bred to have higher levels of tetrahydrocannabinol (THC), the psychoactive component. The higher potency may be even riskier to the teen brain, he added.
Meier noted in a statement that last year was the first time that smoking pot eclipsed smoking tobacco among U.S. teens.
While the researchers found an association between heavy, long-term marijuana smoking and declines in IQ, they did not prove that the marijuana caused the declines.
The U.S. National Institute on Drug Abuse has more on marijuana.
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