About half of Americans use marijuana at some time in their lives, and many are beginning to adolescence. Although some studies suggest that the drug could harm the adolescent brain maturing, the real risk is controversial. Now, in the first study of its kind, scientists have analyzed the consumption of marijuana long term in adolescents, comparing the IQ changes in twins siblings who either used or abstained from marijuana for 10 years . After taking environmental factors into account, the scientists found no measurable link between marijuana use and lower IQ.
"This is a very well-conducted study ... and a welcome addition to the literature," says Valerie Curran, a psychopharmacologist at University College London. She and her colleagues reached "largely the same conclusions "nontwin in a separate study of over 2,000 British teenagers, published earlier this month in the Journal of Psychopharmacology , she said. But, warning that the study has important limitations, George Patton , psychiatric epidemiologist at the University of Melbourne, Australia, added that it does not prove that the particularly heavy marijuana, or chronic use is safe for adolescents.
most studies that linked marijuana for cognitive impairments such as memory loss and low IQ, looked at a single "snapshot" in time, says statistician Nicholas Jackson of Southern California University in Los Angeles, lead author of the new job. This makes it impossible to tell which came first: drug use or poor cognitive performance. "It's a classic chicken egg scenario," he said.
To further probe whether marijuana erodes IQ or inflicts damage in other respects, scientists have begun to follow large groups of drug users among adolescents over time. The first study to do in Dunedin, New Zealand, in 2012 reported significant declines in IQ between 13 and 38 years in heavy users compared to those who used marijuana before age 18 occasionally or not at all. The document "has had a major effect on thinking about the risks of a strong early exposure to cannabis," said Patton, a co-author. Critics, however, noted that the study was not able to rule other possible explanations for the decline in IQ, such as family environment of a teenager or have dropped out.
one way "powerful" to address these concerns is to study identical twins, who share genes and education, said Jackson. in the new study, he and his colleagues looked at 789 pairs of twins teens two courses of studies in the Los Angeles, California, area and the other of Minnesota who enrolled between the ages of 9 and 11. over 10 years, the team administered five tests of intelligence and confidential surveys of marijuana. They also asked about the use of other drugs such as opioid analgesics, cocaine and excessive alcohol consumption.
Marijuana users lost about four IQ points during the study. But their brothers and twin sisters abstainers showed a similar trend of decline, suggesting that the loss of mental acuity was due to something other than pot, said Jackson. "Our results lead us to believe that this" something else "is related to something about the shared environment of twins, which would include the home, school, and peers," he said.
in the new study, adolescents who reported daily use of marijuana for 6 months or more showed no difference in how their IQ has changed, compared to teens who had tried pot less than 30 times. This is a "clear indication that cannabis is unlikely to be the cause of lower IQ," says Claire Mokryz, a PhD student in the laboratory of Curran.
But others say the new study has flaws-especially, a lack of detail on how often and how much adolescents used marijuana. Minnesota groups and Los Angeles have used various surveys on drug use. Questions of the Los Angeles group were much less extensive, Patton said. In surveys administered to this group, for example, participants were asked, "Have you ever tried marijuana?" If a 13 year old respondent answered "yes" after taking one breath, they could be considered as a drug user for each measurement result. "My feeling is that this document is not enough to dismiss the concerns of [our] Dunedin study on the effects of early intensive use of cannabis" in adolescents, Patton said.
Sarah Ewing Feldstein, a psychiatrist at Oregon Health & science University in Portland, agrees. "while it is possible that the results are quite accurate," she said the study represents a "missed opportunity to get a really detailed analysis" of the contribution of cannabis and other substances in IQ.
Although there is "new evidence" that marijuana erodes not IQ, "it does not mean that heavy use in adolescence is no problem," said Jackson. Other aspects of the daily operation might be affected, he said, adding, "we desperately need more research on the effects of marijuana on the brain."
The best way to study the cognitive effects of marijuana would be to administer the drugs to people and see how the duration, frequency and dose affect the brain, said Jackson. "Unfortunately, these types of studies are almost impossible because of federal restrictions," he said. For now, he said, "I am especially concerned about what is happening in the child's environment, who is 14, is seeking refuge in drugs. "
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