A study led by Peter Fried, Barbara Watkinson, Deborah James, and Robert Gray at Carleton University in Ottawa, Canada was conducted to see the effect of marijuana on IQ. In order to gauge marijuana’s effect on the brain the researchers had subjects take an IQ test called the Wechsler Intelligence Scale for Children-III in their preteen years before they started using marijuana. These subjects were split in to 4 groups, those who were heavy users who smoked more than 5 joints a week, light users who smoked less than 5 joints per week, former users who had not smoked regularly in the last 3 months, and non-users who do not smoke more than once per week and did not smoke in the 2 weeks preceding the test. The subjects who took the intelligence test in their preteen years were asked to retake the IQ test which was the Wechsler Adult Intelligence Scale-III. IQ of heavy users had an average decrease of 4.1 points, light users experience gains of 5.8 points, former users also saw gains of 3.5, and non-users had gains of 2.6 points. The researchers concluded that marijuana only has a negative effect on global IQ scores for heavy users. However, the researchers suggested that although marijuana may not have an effect on Global IQ they were not sure if marijuana use could affect memory and attention.

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