"In conclusion, our meta-analysis of studies that have attempted to address the question of longer term neurocognitive disturbance in moderate and heavy cannabis users has failed to demonstrate a substantial, systematic, and detrimental effect of cannabis use on neuropsychological performance. It was surprising to find such few and small effects given that most of the potential biases inherent in our analyses actually increased the likelihood of finding a cannabis effect."
Source
Grant, Igor, et al., "Non-Acute (Residual) Neurocognitive Effects Of Cannabis Use: A Meta-Analytic Study," Journal of the International Neuropsychological Society (Cambridge University Press: July 2003), 9, p. 687.
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