Ian Sample, science correspondent
The Guardian
Tuesday June 3 2008

Smoking cannabis for long periods of time may shrink parts of the brain that govern memory, emotion and aggression, according to researchers in Australia. Scientists used magnetic resonance imaging to scan the brains of people who admitted to smoking more than five joints a day for at least 10 years and compared them with brain images taken from non-drug users.

Those who smoked cannabis regularly had on average a 12% smaller hippocampus, the part of the brain which is thought to be involved with emotion and memory, and a 7% smaller amygdala, which plays a role in regulating fear and aggression.

For the study, researchers imaged the brains of 15 cannabis smokers and 16 individuals who did not use the drug. The scientists, led by Murat Yücel at the University of Melbourne and colleagues at the University of Wollongong, said scans on larger numbers of people were needed to confirm the extent of the effect.

“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,” the scientists report in the journal Archives of General Psychiatry.

Cannabis users also fared worse in tests of verbal memory and were more likely to have low-level symptoms of psychotic disorders such as schizophrenia and mania.

Last month, a team at New York University scanned the brains of a group of 17- to 30-year-olds who had smoked cannabis two to three times a week for at least a year. In that study, the brains of drug users looked no different from those who had never taken cannabis.

In 2004, Cyril D’Souza, a professor of psychiatry at Yale University, reported that THC, the active ingredient in cannabis, caused fleeting schizophrenia-like symptoms in users, ranging from suspiciousness and delusions to poor memory and attention span.

Source: http://www.guardian.co.uk/science/2008/jun/03/drugs.drugsandalcohol