WASHINGTON, Jan. 19 (UPI) -- While past studies have linked marijuana with declines in intelligence, two new studies show the long-held belief may not be true.
The studies, one with British teenagers and the other with American pairs of twins, each show marijuana use during adolescence did not have an effect on later intelligence -- although the British study showed a relationship between smoking cigarettes and poor performance in school.
While previous research has shown marijuana use can affect cognitive performance and intelligence levels, the research failed to account for environmental factors. These other factors, researchers say, do not make marijuana harmless for teenagers.
Researchers at University College London analyzed data on 2,235 teenagers collected as part of the Avon Longitudinal Study of Parents and Children found 24 percent of teens had tried marijuana at least once. Intelligence tests and reviews of educational performance showed little to no difference between teens who used the drug more than 50 times and those who had never used it.
"These findings suggest that adolescent cannabis use is not associated with IQ or educational performance once adjustment is made for potential confounds, in particular adolescent cigarette use," write in the study, published in the Journal of Psychopharmacology. "Modest cannabis use in teenagers may have less cognitive impact than epidemiological surveys of older cohorts have previously suggested."
In another study, published in the Proceedings of the National Academy of Sciences, researchers looked at data from two U.S. studies of twins that included a total of 3,066 individuals who were given intelligence tests between the ages of 9 and 12, and then again between ages 17 and 20.
Researchers found that on average, twins who used marijuana had lower test scores. However, looking at 290 pairs of twins in which one had used marijuana and the other had not, 137 sets of whom were identical and therefore shared DNA, researchers found those who used marijuana did not perform worse than their siblings on tests.
"We found there was no difference between twins in terms of how much their IQ changed," Joshua Isen, a postdoctoral research associate at the University of Minnesota, told HealthDay. "The twin who didn't use marijuana showed as much IQ drop as the twin who did."
The finding suggests teens already at risk for a decline in IQ, due to family, school or other struggles, may be more likely to use pot.
A 2012 study conducted at Duke University suggested heavy marijuana use could affect intelligence all the way into adulthood, however it was later shown to have not considered mental illness, socioeconomic status, or alcohol and cigarette use.