MedWire News: People who have smoked cannabis every day for many years tend to have brain-structure abnormalities and are more likely to experience mild psychosis symptoms than those who have never used the drug, research shows.
"Many people in the community, as well as cannabis users themselves, believe that cannabis is relatively harmless and should be legally available," explain Dr Murat Yücel, from ORYGEN Research Centre in Melbourne, Victoria, Australia, and team.
But they add that some studies have suggested that heavy cannabis use may damage the brain.
Using imaging scans, the researchers studied the brains of 15 cannabis users, aged an average of 40 years, who had smoked at least five joints a day for an average of 20 years, but who had no history of mental health disorders or use of other illegal drugs.
The team also studied the brains of 16 people of similar age and education level who did not use cannabis.
All the participants also underwent memory tests and mental health assessments to test for the presence of mild schizophrenia-like symptoms and other 'subthreshold' psychosis symptoms.
Results of the brain scans revealed that the hippocampus area of the brain, which regulates emotions and memory, was, on average, 12% smaller in the cannabis users than in the other participants. The amygdale area of the brain, which processes fear and aggression, was also smaller in the cannabis users than in the other volunteers.
Cannabis users also had more subthreshold psychosis symptoms and performed worse on the memory test than those who did not use the drug.
Dr Yücel and team conclude: "These results provide new evidence of exposure-related structural abnormalities in the hippocampus and amygdala in long-term heavy cannabis users."
They add: "Although modest use may not lead to significant neurotoxic effects... these findings indicate that heavy daily cannabis use across protracted periods exerts harmful effects on brain tissue and mental health."
The research is published in the Archives of General Psychiatry.