WASHINGTON (Reuters) – Long-term heavy use of marijuana may cause two important brain structures to shrink, Australian researchers said on Monday.

Brain scans showed the hippocampus and amygdala were smaller in men who were heavy marijuana users compared to nonusers, the researchers said. The men had smoked at least five marijuana cigarettes daily for on average 20 years.

The hippocampus regulates memory and emotion, while the amygdala plays a critical role in fear and aggression.

The study, published in the American Medical Association's journal Archives of General Psychiatry, also found the heavy cannabis users earned lower scores than the nonusers in a verbal learning task – trying to recall a list of 15 words.

The marijuana users were more likely to exhibit mild signs of psychotic disorders, but not enough to be formally diagnosed with any such disorder, the researchers said.

"These findings challenge the widespread perception of cannabis as having limited or no harmful effects on (the) brain and behaviour," said Murat Yucel of ORYGEN Research Centre and the University of Melbourne, who led the study.

"Like with most things, some people will experience greater problems associated with cannabis use than others," Yucel said in an e-mail. "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."

Among the 15 heavy marijuana users in the study, the hippocampus volume was 12 percent less and the amygdala volume was seven percent less than in 16 men who were not marijuana users, the researchers said.

The researchers acknowledged that the study did not prove it was the marijuana and not some other factor that triggered these brain differences. But Yucel said the findings certainly suggested marijuana was the cause.

While about half of the marijuana users reported experiencing some form of paranoia and social withdrawal, only one of the nonusers reported such symptoms, Yucel said. The heavy marijuana users, average age 40, said they had used other illicit drugs less than 10 times, the researchers said.

A US group supporting legal sales and regulation of marijuana took issue with the findings, particularly because they were based on men who were such heavy, long-term users.

"These were people who were essentially stoned all day every day for 20 years," Marijuana Policy Project spokesman Bruce Mirken said by e-mail. "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," Mirken added. Yucel said the researchers have begun new research on the effects of both short-term and long-term and moderate and heavy use of marijuana.