Marijuana Use and Cognition
"Current marijuana use had a negative effect on global IQ score only in subjects who smoked 5 or more joints per week. A negative effect was not observed among subjects who had previously been heavy users but were no longer using the substance. We conclude that marijuana does not have a long-term negative impact on global intelligence. Whether the absence of a residual marijuana effect would also be evident in more specific cognitive domains such as memory and attention remains to be ascertained."
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. 887.