http://www.medpagetoday.com/Psychiatry/Addictions/34415
Individuals repeatedly diagnosed with cannabis dependence during young adulthood had noticeable declines in IQ scores by age 38, especially when the heavy use started in their teens, researchers said.
A small to medium decline in mean IQ between tests taken on the eve of adolescence and again at age 38 was seen in those diagnosed at least three times with cannabis dependence, according to Madeline Meier, PhD, of Duke University, and colleagues, who reported on data from 1,037 participants in a New Zealand birth cohort.