June 2 (HealthDay News) -- People who use marijuana for a long time can develop abnormalities in their brains, Australian researchers report.

Although growing literature suggests that long-term marijuana use is associated with a wide range of adverse health consequences, many people believe it is relatively harmless and should be legalized, the researchers noted.

"However, this study shows long-term, heavy cannabis use causes significant brain injury, memory loss, difficulties learning new information, and psychotic symptoms, such as delusions of persecution [paranoia], delusions of mind-reading, and bizarre social behaviors in even non-vulnerable users," said lead researcher Murat Yucel, from the ORYGEN Research Centre and the Neuropsychiatry Centre at the University of Melbourne.

This new evidence plays an important role in further understanding the effects of marijuana and its impact on brain functioning, Yucel said. "The study is the first to show that long-term cannabis use can adversely affect all users, not just those in the high-risk categories such as the young, or those susceptible to mental illness, as previously thought," he said.

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

In the study, Yucel's team did high-resolution MRIs on 15 men who smoked more than five joints a day for more than 10 years. They compared those with scans of 16 men who did not use marijuana.

In addition, all the men took verbal memory tests and were examined for symptoms of psychiatric disorders.

"The more marijuana used, the more these individuals were likely to show reduced brain volumes in the hippocampus and amygdala, as well as being more likely to develop symptoms of psychotic disorders and to have significant memory impairment," Yucel said.

In fact, the hippocampus of marijuana users was 12 percent smaller, and the amygdala was 7.1 percent smaller than among nonusers. In addition, men who used marijuana also had symptoms of psychiatric disorders, Yucel's group found.

The hippocampus is associated with the regulation of emotion and memory, while the amygdala controls fear and aggression.

"There is ongoing controversy concerning the long-term effects of cannabis on the brain," Yucel said. "These findings challenge the widespread perception of cannabis as having limited or no harmful effects on brain and behavior. Although modest use may not lead to significant neurotoxic effects, these results suggest that heavy daily use might indeed be toxic to human brain tissue."

One expert agrees that heavy marijuana use can have negative effects on the brain.

"These findings are not surprising," said Dr. Adam Bisaga, an assistant professor of psychiatry at Columbia University and an addiction psychiatrist at New York State Psychiatric Institute. "Chronic use of large amounts of any substance that is affecting neural transmission will most likely invoke adaptive changes and lead to the reorganization of neural networks, and possibly affect brain structures."

Heavy users of marijuana probably represent only a very small proportion of users, Bisaga said.

"It is not clear if any clinically significant changes can be seen in recreational, infrequent marijuana users, who were not studied here. These findings suggest that public health education, as well as screening, early recognition, and treatment of cannabis dependence, may prevent the progression of the disease and the loss of brain function and related psychiatric complications," Bisaga said.

SOURCES: Murat Yucel, Ph.D., ORYGEN Research Centre, Melbourne Neuropsychiatry Centre, University of Melbourne, Australia; Adam Bisaga, M.D., assistant professor, psychiatry, Columbia University, and addiction psychiatrist, New York State Psychiatric Institute, New York City; June 2008, Archives of General Psychiatry