Scientists have found that smoking weed does not make you stupid after all

No evidence that adolescent marijuana use leads to a decline in intelligence
The Washington Post (US)
Monday, January 18, 2016

A 2012 Duke University study found that persistent, heavy marijuana use through adolescence and young adulthood was associated with declines in IQ. Other researchers have since criticized that study's methods. A follow-up study found that the original research failed to account for a number of confounding factors, such as cigarette and alcohol use, mental illness and socioeconomic status. Two new reports tackle the relationship between marijuana use and intelligence from two very different angles: One examines the life trajectories of 2,235 British teenagers between ages 8 and 16, and the other looks at the differences between American identical twin pairs in which one twin uses marijuana and the other does not.