London - Young people who smoke cannabis for years run the risk of a significant and irreversible reduction in their IQ, according to findings from a study of around 1,000 people in New Zealand.
An international team found those who started using cannabis below the age of 18, while their brains were still developing, suffered a drop in IQ.
Professor Terrie Moffitt, a researcher at the Institute of Psychiatry, King's College London, said that "those who started using cannabis regularly when they were in secondary school had lost, on average, about eight IQ points". She also said this study could have a significant impact on our understanding of the dangers posed by cannabis use.

A British expert said the research might explain why people who use the drug often seem to under-achieve.
For more than 20 years researchers have followed the lives of a group of people from Dunedin in New Zealand. They assessed them as children, before any of them had started using cannabis, and then re-interviewed them repeatedly, up to the age of 38.
Having taken into account other factors such as alcohol or tobacco dependency or other drug use, as well the number of years spent in education, 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 people smoked, the greater the loss in IQ. The effect was most marked in those who started smoking cannabis as adolescents.
Stopping or reducing cannabis use failed to fully restore the lost IQ.
The researchers, writing in the US journal Proceedings of the National Academy of Sciences, found that: "Persistent cannabis use over 20 years was associated with neuropsychological decline, and greater decline was evident for more persistent users."