Peter Fried and colleagues report that light and former use of marijuana does not appear to have a long-term effect on intelligence, while heavy use appears to be detrimental.
Fried and colleagues followed 70 subjects in the Ottawa Prenatal Prospective Study, and compared intelligence quotient (IQ) scores of subjects at 9–12 years of age (before initiation of marijuana use) with their scores at 17-20 years. The authors grouped subjects as nonusers (n= 37), light users (less than 5 joints per week, n=9), former users of marijuana (no marijuana use in at least 3 months, n=9) or heavy users.
The authors found that among heavy users (more than 5 joints per week, n=15) IQ scores decreased by 4.1 points on average, while gains in IQ scores were seen among light users (mean 5.8 points), former users (mean 3.5 points), and nonusers (mean 2.6).
The authors state that while there was a significant decline in IQ scores, the scores of the subjects — at a mean of 109.1 — were still above average at the young adult assessment (mean 105.1). They add that if preteen IQ had not been assessed, the subjects would have appeared to be functioning normally. The authors suggest further investigation into the cognitive consequences of both current and previous marijuana use, particularly since the popularity of the drug has been increasing over the last 4 years.
Current and former marijuana use: preliminary findings in a longitudinal study of effects on IQ in young adults
— P. Fried
Canadian Medical Association Journal