The findings come 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. 

 

A UK 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 persistently² used cannabis 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. 

 

For example, researchers found that individuals who started using cannabis in adolescence³ and then carried on using it for years showed an average eight-point IQ decline.

 

Stopping or reducing cannabis use failed to fully⁴ restore the lost IQ. 

 

The researchers, writing in the US journal Proceedings⁵ of the National Academy of Sciences, found that: "Persistent¹ cannabis use over 20 years was associated with neuropsychological decline, and greater decline was evident for more persistent users." 

 

"Collectively, these findings are consistent with speculation⁶ that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic（毒害神经的） effects." 

 

One member of the team, Prof Terrie Moffitt of King's College London's Institute of Psychiatry⁷, said this study could have a significant impact on our understanding of the dangers posed by cannabis use.

 

"This work took an amazing scientific effort. We followed almost 1,000 participants, we tested their mental abilities as kids before they ever tried cannabis, and we tested them again 25 years later after some participants became chronic⁸ users.

点击收听单词发音