A new study from New Zealand concluded that "young people who smoke [marijuana] for years run the risk of a significant and irreversible reduction in their IQ." An international team of researchers studied around 1,000 people who regularly used marijuana. The team assessed the drug users as children before they had started using pot, and then re-interviewed them over a 20 year period after the study participants had started smoking pot.

An article from the BBC summarized the results of the study:

They found that those who persistently used cannabis—smoking it at least four times a week year after year through their teens, 20s and, in some cases, their 30s—suffered a decline in their IQ. The more that people smoked, the greater the loss in IQ. The effect was most marked in those who started smoking cannabis as adolescents.

A researcher not associated with the study concluded, "It is of course part of folk-lore among young people that some heavy users of cannabis … seem to gradually lose their abilities and end up achieving much less than one would have anticipated. This study provides one explanation as to why this might be the case."