People who regularly use cannabis from their teens may suffer memory loss and a lower IQ by the time they reach their late 30s, an international study of 1000 people in Dunedin has found.
The research published by the Proceedings of the National Academy of Sciences in the United States examined the decline in IQ and cognitive abilities by testing the Dunedin people, born in 1972/73, at age 13 and again at age 38.
Study members who regularly used cannabis showed a greater decline in IQ compared with other participants and their habit appeared to affect everyday cognitive functioning.
Regular use is defined as four times a week.
Teenage cannabis users tended to become regular users and experienced a greater IQ decline compared with those who started using cannabis as adults.
Quitting or reducing cannabis use did not fully restore brain functioning among teenage cannabis users, and the researchers say cannabis use in adolescence, when the brain is developing, could have "neurotoxic effects".
The study was led by Madeline Meier of Duke University in North Carolina and included other researchers from the US, the United Kingdom and Otago University in Dunedin.
Commenting on the findings, Dr Simon Adamson, senior lecturer at Otago University's national addiction centre in Christchurch, says the magnitude of cognitive decline from heavy regular cannabis use is of real concern.
"Clearly we must focus energy on reducing the prevalence of cannabis use in adolescence," he said.
Robin Murray, professor of psychiatric research at King's College London, says it's "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," he said.
But the study did not address the type of cannabis used.
"We now know that use of high potency types of cannabis (called skunk in the UK) appears to be more likely to cause problems," he said.