Reefer madness debunked as major UK study finds marijuana does not reduce IQ in teens
A young man smoking at the Vancouver Global Marijuana March 2015 (Danny Kresnyak/Cannabis Culture)

A large study conducted in the United Kingdom failed to find evidence of a robust link between cannabis use and lowered intelligence among teenagers.

The study, published online January 6 in the Journal of Psychopharmacology, found a statistical association between cannabis use and decreased intellectual performance. However, this association vanished when the researchers took other variables into account. “The notion that cannabis use itself is causally related to lower IQ and poorer educational performance was not supported in this large teenage sample,” wrote Claire Mokrysz of the University College London and her colleagues.

The findings are based on 2,235 teenagers who participated in the Avon Longitudinal Study of Parents and Children (ALSPAC), a long-term study following children born in the Bristol area in 1991 and 1992. The children had their IQ tested at the age of 8, and again at the age of 15. Nearly a quarter of the participants reported having tried cannabis at least once, and 3.3 percent had used the drug at least 50 times.

The teenage cannabis users had lower IQ scores and worse educational performance. But the researchers found no evidence that cannabis itself was to blame for the correlation. Teenage cannabis users were also more likely to have had problems in childhood and to have used other drugs, such as cigarettes and alcohol.

“After adjustment to account for these group differences, cannabis use by the age of 15 did not predict either lower teenage IQ scores or poorer educational performance,” the researchers explained. “These findings therefore suggest that cannabis use at the modest levels used by this sample of teenagers is not by itself causally related to cognitive impairment. Instead, our findings imply that previously reported associations between adolescent cannabis use and poorer intellectual and educational outcomes may be confounded to a significant degree by related factors.”

The heaviest cannabis users did appear to suffer some negative consequences, but the effects were small. Teenagers who had used cannabis more than 50 times had an IQ score that was 0.1 point lower than never-users on average. These teens also had marginally poorer exam results.

The findings clash with a previously published study that found frequent cannabis users could lose an average of six IQ points between age 13 and age 38. That study, published in 2012 in the journal PNAS, was based on a longitudinal study of 1,037 New Zealanders.

Mokrysz, the lead author of the new study, said researchers need to take many factors into account when examining the effect of cannabis on intelligence.

“Our findings suggest cannabis may not have a detrimental effect on cognition, once we account for other related factors,” she said in a news release that was published before her research was presented the annual congress of the European College of Neuropsychopharmacology (ECNP) in 2014. “This may suggest that previous research findings showing poorer cognitive performance in cannabis users may have resulted from the lifestyle, behaviour and personal history typically associated with cannabis use, rather than cannabis use itself.”

“People often believe that using cannabis can be very damaging to intellectual ability in the long-term, but it is extremely difficult to separate the direct effects of cannabis from other potential explanations,” Mokrysz added. “Adolescent cannabis use often goes hand in hand with other drug use, such as alcohol and cigarette smoking, as well as other risky lifestyle choices. 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.”

The article "Major UK study finds using cannabis does not reduce IQ among teenagers" was originally published at PsyPost.