One of the most thorough studies of its kind has concluded that smoking too much pot as a teenager is linked to significant cognitive decline. The study followed 1,037 New Zealanders from birth through age 38, monitoring their IQs and marijuana usage throughout. The findings reveal that test subjects who smoked heavily throughout adolescence suffered an average drop of eight points in their IQ scores. According to Madeline H. Meier, first author of the study, this impairment was most prevalent among adolescent-onset cannabis users:
http://www.pnas.org/content/early/2012/08/22/1206820109.abstract
http://www.kwtx.com/news/health/headlines/Teenage-Pot-Use-Linked-To-Later-IQ-Declines-167591465.html
