“I have built my career, the last 20 years, on the study of how marijuana affects the brain, and am very familiar with the existing data on negative outcomes. Effects on IQ are absolutely not seen in all studies. It has really only been correlated with long-term, heavy, daily use outside of the context of debilitating disease, which does not reflect the medicinal user.
Another study recently published in the Journal of Psychopharmacology, a sample of 2,235 children had their cognitive abilities tested at eight years old and then at 15 and 16 years old. Those who had used cannabis more than 50 times did not statistically differ from never-users on either IQ or educational performance.”