Recent studies indicate that fewer adolescents believe that regular cannabis use is harmful to health. The authors of this study examined whether there is an association between persistent cannabis use and neuropsychological decline. Participants were 1,037 individuals from the Dunedin Study, followed from birth (1972/1973) to age 38 years. Cannabis use was determined in interviews at ages 18, 21, 26, 32, and 38 years. Neuropsychological testing was conducted at age 13 years, before initiation of cannabis use, and again at age 38 years, after a pattern of persistent cannabis use had developed.

The results indicated that persistent cannabis use was associated with neuropsychological decline across multiple domains of functioning, even after controlling for years of education. Impairment was concentrated among adolescent-onset cannabis users, and more persistent use was associated with greater decline. Cessation of cannabis use did not fully restore neuropsychological functioning among adolescent-onset cannabis users. The authors suggest that cannabis may have a neurotoxic effect on the adolescent brain. They highlight the importance of prevention efforts targeting adolescents.

Meier MH, Caspi A, Ambler A, Haarington HL, Houts R, Keefe RSE, McDonald K, Ward A, Poulton R and Moffitt TE: Persistent cannabis users show neuropsychological decline from childhood to midlife. Proc. Natl. Acad. Sci. USA [Epub ahead of print August 27, 2012, doi: 10.1073/pnas.1206820109]

http://www.pnas.org/content/early/2012/08/22/1206820109