Regular dope smokers experience serious cognitive impairment long after they're high, an Aussie expert says.
University of Wollongong Associate Professor Nadia Solowij said memory, attention and higher cognitive functions were all affected in the long-term by heavy use.
"The longer you use cannabis and the more heavily you use cannabis, and the younger you are when you start using cannabis, the greater the changes that we see in the actual structure of the brain," Ms Solowij said.
"The adolescent brain is more sensitive to insult from drugs, and we've been finding that the younger people start using, the worse the cognitive impairment.
"There are cognitive impairments that persist beyond the period of acute intoxication, and these are largely in the domains of memory, attention and higher cognitive functions."
Ms Solowij, who has been researching the effects of cannabis on the brain for more than 20 years, said her research has focused on its relationship with schizophrenia and other mental health issues.
She will be speaking at a cannabis and health symposium being held in Auckland next week.
"There are millions of cannabis users worldwide but clearly they don't all develop schizophrenia so we're trying to understand what makes a person vulnerable to developing schizophrenia if they use cannabis, and by what mechanisms might cannabis trigger psychosis," she said.
Ms Solowij said she was concerned about the younger age at which people were starting to use cannabis.
"It's one thing for adults to choose to do what they do recreationally, but with young people - where the brain is developing in really critical ways - it's really important to try to understand what's going on there when they use cannabis.
"Millions of them will end up not developing anything really major, but for those who do develop a psychosis it's very tragic.
"I think it's really important to understand why that happens to some individuals and others can go on happily smoking for many years."
The three-day symposium is being held by the New Zealand Drug Foundation and starts on Wednesday.
Foundation executive director Ross Bell said Ms Solowij was at the cutting edge of research about the cognitive effects of cannabis.