MONDAY, Jan. 18, 2016 (HealthDay News) -- Studies have suggested that teenage marijuana use leads to a decline in intelligence. But new research with twins suggests the link may not be as clear-cut as some believe.
Confounding family issues that lead a kid to try pot in the first place may be to blame for any brain drain, according to the new study.
The researchers did report that marijuana users had lower IQ test scores, on average, when compared to those who didn't indulge in pot.
But they also discovered that a twin who uses pot winds up with about the same IQ as a twin who doesn't.
"We found there was no difference between twins in terms of how much their IQ changed," said study co-author Joshua Isen, a postdoctoral research associate at the University of Minnesota. "The twin who didn't use marijuana showed as much IQ drop as the twin who did."
These results suggest that people who think marijuana causes IQ decline may be putting the cart before the horse, said Dr. Joseph Lee, medical director for the Hazelden Betty Ford Foundation's youth services.
Kids likely to experience a decline in IQ for whatever reason -- be it family problems, emotional turmoil or some other factor -- appear more likely to try marijuana, drink alcohol or smoke cigarettes at an early age, Lee said.
"Certain kids are at higher risk for problems down the road," Lee said. "For some reason, groups of kids who tend to adopt heavier substance use at an earlier age just do worse at life."
Marijuana use should be a red flag to parents that their kid is struggling and needs help at home and in school, Isen and Lee said.
"This is a big red flag for their life's trajectory," Lee said.
About one in five older teens and young adults has used marijuana in the past month, the researchers said in background notes. Previous research has raised concerns that marijuana use could affect the structure and function of the still-developing teenage brain.
Teen pot smoking has been associated in earlier studies with decreased intelligence, reduced memory, poorer attention and lower verbal ability.
To determine whether marijuana is causing these problems, or is simply a warning sign of deeper troubles, the researchers behind the new study decided to examine pot use among more than 3,000 teenage twins.
They focused on twins because the kids have the same background and share most or all of their genes, depending on whether they are identical or fraternal twins, Isen said.
"In one fell swoop, that allowed us to control for common familial variables and shared genes," he said.
The researchers tested the twins' IQ at two points of their lives -- between the ages of 9 and 12, before either would likely become involved with pot, and again at ages 17 to 20.
Comparing all marijuana users to all nonusers, the researchers found that the average change in IQ was 3.4 to 4 points lower in teens who had used pot.
But twins who smoked pot did not exhibit a change in IQ significantly different than that experienced by their non-using twin.
"This study's conclusions are consistent with those of previous, well-controlled trials finding that cannabis exposure possesses little if any neurotoxicity risk, and that its cumulative consumption is not adversely associated with IQ or academic performance once investigators account for potential confounding variables, such as cigarette smoking, binge drinking or poor family environment," said Paul Armentano. He is deputy director of NORML, which advocates for reform of marijuana laws.
Isen agreed that these kids' problems may trace initially to their family or their emotional makeup, rather than pot or alcohol use.
"It's the type of family that have kids who initiate marijuana use as teenagers that seem to be on that trajectory for intellectual stagnation," he said. "Teenagers who use marijuana tend to be more delinquent and rule-breaking in general -- so maybe they aren't reading, they aren't doing their homework, they're skipping classes."
The study was published Jan. 18 in the Proceedings of the National Academy of Sciences.
The U.S. National Institute on Drug Abuse has information for parents about pot use.