The persistent use of cannabis before age 18 has been linked to lasting harm to intelligence, according to a large study.
Analysis of more than 1000 New Zealanders found those who took up cannabis in adolescence and used it for years afterwards experienced an average decline in IQ of eight points when measured at age 13 and 38.
People who did not begin using cannabis until they were adults, with fully formed brains, did not show the same declines.
Experts here and abroad say the findings are significant and could offer some explanation for the "teenage stoner" stereotype.
Lead researcher Madeline Meier of Duke University in Durham, North Carolina, said quitting cannabis later in life did not appear to reverse the loss of intelligence.
Higher IQ correlated with higher education and income and better health, she said.
"Somebody who loses eight IQ points as an adolescent may be disadvantaged compared to their same-age peers for years to come."
The study, to be published in the Proceedings of the National Academy of Sciences, examined individuals in the Dunedin Cohort study, which has followed 1037 people born in 1972-73 in Dunedin from birth.
About 5 per cent of the study group were considered cannabis-dependent, or used the drug more than once a week before age 18.
At age 38, all of the study participants were given a range of psychological tests to assess memory, processing speed, reasoning and visual processing. People who smoked cannabis persistently as teens scored significantly worse on most tests.
The decline in IQ among teenage users could not be explained by alcohol, other drug use or having less education, the team of international researchers found.
Dr Meier said a drop of eight IQ points was significant.
NZ and international experts have said the research provides valuable insight into the harm that could be caused by cannabis.
"Clearly we must focus energy on reducing the prevalence of cannabis use in adolescence," said Dr Simon Adamson, of Otago University's National Addiction Centre.
Ross Bell, head of the NZ Drug Foundation, said a clear picture had emerged that drugs such as cannabis and alcohol were particularly damaging for adolescents.
"The worrying thing in New Zealand is that young New Zealanders have pretty easy access to alcohol and cannabis and substances like butane."
Simply banning drugs such as cannabis and then thinking that was the problem solved would not work, Mr Bell said. "What we're lacking in New Zealand is support for widespread, high-quality, well-constructed prevention messages targeted at younger people."
New Zealanders are among the highest users of illegal drugs in the world, and top the list for cannabis use, according to a United Nations study released in June.
Robin Murray, professor of psychiatric research at King's College London, said the study could offer some explanation for the "teenage stoner" stereotype.
"It is of course part of folklore among young people that some heavy users of cannabis - my daughter calls them stoners - seem to gradually lose their abilities and end up achieving much less than one would have anticipated."By Nicholas Jones Email Nicholas