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, “[T]hose 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.”
A widely publicized New Zealand study published in 2012 in The Proceedings of the National Academy of Sciences 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 socioeconomic differences, not cannabis use.
More recently, the results of a 2015 study in the journal Drug and Alcohol Dependence reported that the effects of persistent adolescent cannabis use on academic performance “became non-significant after controlling for persistent alcohol and tobacco use.”
Full text of the study, “Are IQ and educational outcomes in teenagers related to their cannabis use? A prospective cohort study,” appears online in The Journal of Pharmacology.