Brain scans show that the hippocampus and amygdala in 15 individuals who smoked five or more marijuana cigarettes daily for more than 20 years were smaller than those of nonusers, Reuters reported June 3.

The two brain regions are involved in memory and emotion and fear and aggression, respectively.

Researchers also found that chronic, long-term marijuana users performed more poorly on memory tests and exhibited signs of mild psychotic disorders. "Our findings suggest that everyone is vulnerable to potential changes in the brain, some memory problems and psychiatric symptoms if they use heavily enough and for long enough," said Murat Yucel of the ORYGEN Research Centre and the University of Melbourne in Australia.

"These were people who were essentially stoned all day every day for 20 years," said Marijuana Policy Project spokesman Bruce Mirken. "This study says nothing about moderate or occasional users, who are the vast majority -- and the (study) even acknowledges this. The documented damage caused by comparably heavy use of alcohol or tobacco is just off-the-charts more serious, and you don't need high-tech scans to find it."

The study was published in the June 2008 issue of the Archives of General Psychiatry.