Persistent marijuana use beginning in adolescence is associated with a decline in cognitive function by midlife, according to a study published in the Proceedings of the National Academy of Sciences (Meier MH et al. Proc Natl Acad Sci. doi: 10.1073/pnas.1206820109 [published online August 27, 2012]).
Heavy use of marijuana has been previously linked to neuropsychological impairment, particularly when use begins early in life. But much of this evidence came from case-control studies, which may be vulnerable to certain biases. The latest study followed up 1037 individuals in New Zealand from their births in 1972 or 1973 until age 38 years. Participants underwent neuropsychological testing at age 13 years to develop a baseline and were tested again at age 38. Interviews were conducted with patients at ages 18, 21, 26, 32, and 38 years to assess cannabis use.
Kuehn BM. Marijuana Use Starting in Youth Linked to IQ Loss. JAMA. 2012;308(12):1196. doi:10.1001/2012.jama.12205