Regular cannabis use during adolescence does lasting neuropsychological harm

Marijuana use during young adulthood has been thought to have neurotoxic effects to which the developing brain is particularly sensitive. A study just published in the Proceedings of the National Academy of Sciences (of the USA) offers evidence that regular marijuana use results in neuropsychological decline from childhood to midlife. The authors observe:

"Recent reports show that fewer adolescents believe that regular cannabis use is harmful to health. Concomitantly, adolescents are initiating cannabis use at younger ages, and more adolescents are using cannabis on a daily basis. The purpose of the present study was to test the association between persistent cannabis use and neuropsychological decline and determine whether decline is concentrated among adolescent-onset cannabis users."

They studied marijuana use among 1,037 individuals who were followed from birth in 1972/1973 to 38 years of age. The subjects were interviewed at ages 18, 21, 26, 32, and 38 years to gauge consumption; and examined by neuropsychological testing at age 13 years before starting to use pot, and again at age 38 years after a persistent pattern of use use had developed. The data showed significant neuropsychological harm:

"Persistent cannabis use was associated with neuropsychological decline broadly across domains of functioning, even after controlling for years of education. Informants also reported noticing more cognitive problems for persistent cannabis users. Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline. Further, cessation of cannabis use did not fully restore neuropsychological functioning among adolescent-onset cannabis users."

Marijuana may have merit for alleviating pain and nausea in cancer care. It is not, however, a benign pharmaceutical substance. Like any agent that modifies brain function, persistent use over time results in compensatory changes that sacrifice normal brain physiology. That cessation of cannabis use did not fully restore neuropsychological function is particularly disturbing. The authors conclude:

"Findings are suggestive of a neurotoxic effect of cannabis on the adolescent brain and highlight the importance of prevention and policy efforts targeting adolescents."