HABITUALLY SMOKING CANNABIS as a teenager could affect a person’s IQ, according to new research.

A study that tracked more than 1,000 adolescents in New Zealand from youth into middle age found those who began habitually smoking cannabis before the age of 18 experienced up to an eight-point drop in IQ between the ages of 18 and 38. The average IQ is 100 points and a drop of eight represents a fall from the 50th percentile to the 29th.

The IQs of all study subjects were measured at the age of 13, before habitual cannabis use began, and cannabis use was then assessed at five “waves” at the ages of 18, 21, 32 and 38, with the researchers controlling for the use of other drugs and education levels.

Those who experienced an eight-point drop in IQ were those who began smoking in adolescence and engaged in “habitual smoking” – ie using cannabis more than four days a week – during three “waves” or more.

Those who began smoking in adolescence but used cannabis less persistently experienced a drop in their IQs, but the drop was less pronounced than in those who had used cannabis early and persistently, according to the New York Times.

People who did not smoke until after adolescence showed no adverse effects on intelligence, according to the study.

“Collectively, these findings are consistent with speculation that cannabis use in adolescence, when the brain is undergoing critical development, may have neurotoxic effects,” wrote Madeline Meier, lead researcher and a post-doctoral associate at Duke University.

The research appears in Proceedings of the National Academy of Sciences.