Cannabis use by adolescents is not associated with lower IQ or poorer educational performance once adjustments are made for potential confounders, specifically cigarette smoking, according to data published in the Journal of Psychopharmacology.
British investigators assessed the relationship between cumulative cannabis use and IQ at the age of 15 and educational performance at the age of 16 in a cohort of 2,235 adolescents.
After researchers adjusted for potentially confounding variables, such as childhood depression and cigarette use, they reported: "Those who had used cannabis greater than or equal to 50 times did not differ from never-users on either IQ or educational performance."
By contrast, teen cigarette smoking was associated with poorer educational outcomes even after researchers adjusted for other confounding variables.
Researchers concluded: "In summary, the notion that cannabis use itself is causally related to lower IQ and poorer educational performance was not supported in this large teenage sample."
No effect on academic performance
A widely publicized New Zealand study published in 2012 reported that frequent use of cannabis by those under the age of 18 was associated with lower IQ by age 38. However, a separate review of the data published later in the same journal suggested that the changes were likely the result of socio-economic differences, not cannabis use.
Full text of the study, “Are IQ and educational outcomes in teenagers related to their cannabis use? A prospective cohort study,” appears online here.