Adolescent cannabis use may affect adult IQ

By Brandon Smith

Researchers at Duke University have found a possible link between IQ deterioration and smokingmarijuana at an early age.

The study, which asserted that persistent cannabis users show neuropsychological decline from adolescence to middle age, was published Aug. 27 in the Proceedings of the National Academy

of Sciences.

Lead author Madeline Meier said she focused on a 25-year-long study conducted in Dunedin, New Zealand. The study followed more than 1,000 13-year-olds until they turned 38.

The participants’ neuropsychological health was tested at various ages, starting before any of them began using cannabis.

“This study offered us a unique opportunity to test the hypothesis that cannabis use is associated with decline [in IQ],” Meier said. “It is consistent with a bunch of research showing that the adolescent brain might be particularly vulnerable to the effects of toxic


According to co-author Terrie Moffitt, previous studies have struggled to prove such claims because it is difficult to rule out other factors that may affect IQ.

“We showed that regardless of their initial [pre-cannabis] test performance, adolescentonset cannabis users performed worse than non-users,” Moffitt said.

According to researchers, people who started using cannabis at earlier ages showed consistent declines in their IQ—up to 8 points—which, according to the authors, is the equivalent of a student dropping from an A to a B average.

Although the authors of the paper acknowledged there are other factors that may contribute to the decline in intelligence, they believe most were ruled out.

Louis Silverstein, distinguished professor in Columbia’s Department of Humanities, History

and Social Sciences disagrees.

“These studies are easy to criticize because it is impossible to isolate particular substances,” Silverstein said. “How do you take a group of kids and get them to say, ‘OK, for the next year we’re only going to smoke marijuana?’”

According to Silverstein, there are other significant implications when reading studies like Meier’s.

“Who’s funding this kind of research?” he asked. “There are so many chemicals and substances, both legal and illegal, that people ingest. Nobody can say that cannabis has a direct link to such findings. Marijuana has always fallen under this sort of scrutiny.”

Meier said measures were taken to protect the integrity of the study’s results.

“Anybody who had, for example, been dependent on alcohol, drugs or was diagnosed with schizophrenia had been taken out of the analysis,” she said. “We also looked at people’s educational levels. We showed that people with similar educational statuses still had associations with IQ decline and cannabis use.”

What is most important to take away from this study is that marijuana is a potentially dangerous substance to the developing mind just like alcohol and other drugs, Meier said. But more research is needed before any real conclusions can be made regarding the potential benefits or harmful effects of cannabis use.

“Given that the brain undergoes dynamic changes from the onset of puberty through early adulthood, this developmental time should be the focus of future research,” she said. “These findings are also limited to a cohort group of individuals in New Zealand, and additional research is needed to see if these effects occur in different countries and in today’s teenagers.”