Among more than 2,200 U.K. children who took IQ tests at age 8 and at age 15, marijuana use in the teen years appeared to be associated with lower IQ scores, the researchers said.
However, the researchers found a strong link between marijuana use and other risky behaviors such as alcohol, cigarette and drug use. When these other behaviors were taken into account, there was no connection between marijuana use and lower IQ at age 15, they said.
The study did find that heavy marijuana users (at least 50 times by age 15) scored an average of 3 percent lower on compulsory school exams taken at age 16.
The findings were presented this week at a meeting of the European College of Neuropsychopharmacology in Berlin, Germany.
"Our findings suggest cannabis may not have a detrimental effect on cognition, once we account for other related factors -- particularly cigarette and alcohol use," said lead researcher Claire Mokrysz, of University College London.
"This may suggest that previous research findings showing poorer cognitive performance in cannabis users may have resulted from the lifestyle, behavior and personal history typically associated with cannabis use, rather than cannabis use itself," she said in an association news release.
Teenage cannabis use frequently goes hand in hand with other drug use, such as alcohol and cigarette smoking, as well as other risky lifestyle choices, she added. "It's hard to know what causes what -- do kids do badly at school because they are smoking weed, or do they smoke weed because they're doing badly? This study suggests it is not as simple as saying cannabis is the problem," Mokrysz explained.
She concluded: "This is a potentially important public health message -- the belief that cannabis is particularly harmful may detract focus from and awareness of other potentially harmful behaviors."
However, she said the finding that heavier cannabis use is linked to marginally worse educational performance is important and warrants further investigation.
Guy Goodwin, chair of the European College of Neuropsychopharmacology, said this is a "potentially important study." It suggests that the current focus on the alleged harms of marijuana may obscure the fact that its use is often correlated with use of other available drugs and possibly lifestyle factors, he explained.
"These may be as or more important than cannabis itself," Goodwin, a professor at Oxford University, said in the news release.
Data and conclusions presented at meetings are usually considered preliminary until published in a peer-reviewed medical journal.
The U.S. National Institute on Drug Abuse has more about marijuana.