"Although the heavy current users experienced a decrease in IQ score, their scores were still above average at the young adult assessment (mean 105.1). If we had not assessed preteen IQ, these subjects would have appeared to be functioning normally. Only with knowledge of the change in IQ score does the negative impact of current heavy use become apparent."
Source
Fried, Peter, Barbara Watkinson, Deborah James, and Robert Gray, "Current and former marijuana use: preliminary findings of a longitudinal study of effects on IQ in young adults," Canadian Medical Association Journal, April 2, 2002, 166(7), p. 890.
http://www.ncbi.nlm.nih.gov...
