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."

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. 887.
http://www.ncbi.nlm.nih.gov...

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