Robert Gonzalez goes over the results of a new study regarding the effects of marijuana use on the IQs of adolescents:
The study followed 1,037 New Zealanders from birth through age 38, monitoring their IQs and marijuana usage throughout. IQ tests were administered first at age 13 (before the test subjects had started smoking pot), and again at 38, "after a a pattern of persistent cannabis use had developed." 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[.]
Gonzalez distills the findings:
[T]he question of "how much" [use] is arguably less important than "when," or "what age?" The fact that cognitive impairment was detected in adolescent-onset smokers and not adult-onset smokers is highly suggestive that weed has a neurotoxic effect on the developing adolescent brain (what Meier and her colleagues refer to as the "developmental vulnerability hypothesis"), a conclusion that gels with a number of previous studies.