There is conflicting evidence regarding the long-term effects of regular cannabis use. Although there is a growing literature suggesting that long-term cannabis use is associated with a wide variety of health problems many people in the community, as well as cannabis users themselves, believe it is relatively harmless and should be legally available. Animal studies have shown that long-term cannabis administration can affect the hippocampus but only a handful of brain-imaging studies have been undertaken in humans. An Australian study compared fifteen long-term cannabis users with a sixteen-strong control group and found that the hippocampi and amygdalas of the cannabis users were significantly smaller. The more people had smoked cannabis the smaller their hippocampi were and the more likely they were to have 'sub-threshold' psychosis symptoms. Positive symptom scores - delusions, hallucinations etc - were also associated with cumulative exposure to cannabis.
Yucel, Murat ... [et al] - Regional brain abnormalities associated with long-term heavy cannabis use Archives of General Psychiatry June 2008, 65(6), 694-701