Cannabis, or marijuana, is the most widely used illicit drug in the developed world. There is perception that cannabis does not have toxic effects on the brain, but research on the long-term effect of cannabis on the brain is scarce. In the June 2008 issue of Archives of General Psychiatry, researchers from Australia reported that long-term, heavy cannabis use is linked to structural abnormalities in the brain.

The investigators recruited 15 men from the general community who had used more than 5 joints of cannabis daily for more than 20 years. The recruited participants had no history of polysubstance abuse or neurologic/mental disorder. The investigators performed imaging of the brain and examined two regions of the brain - the hippocampus and the amygdala - that have high concentrations of receptors for cannabis and cannabis-like substances. They found that, when compared to control subjects who did not use cannabis, the long-term heavy cannabis users had smaller hippocampal and amygdala volumes.

It is not known exactly what these structural abnormalities mean, but the investigators suggested that reductions in hippocampal volume are consistent with the view that cannabis use increases the risk of psychotic symptoms. Although larger prospective studies are needed, these findings suggest that heavy daily cannabis use over a long period of time may exert harmful effects.