Australian scientists claim to have found a correlation between heavy cannabis smoking and decreased brain size in a recent study. The study was carried out at the University of Melbourne with help from members of the University of Wollongong.

The context of the study cited that “Cannabis is the most widely used illicit drug in the developed world… there is a paucity of research examining its long-term effect on the human brain.” With this in mind, researchers set out to discover if there was any effect on the two most “cannabinoid receptor-rich regions of the brain”, namely the hippocampus (the part of the brain asociated with emotion and memory) and the amygdala (the section which plays a role in regulating fear and aggression.)

The study was carried out on a small section of the general community, including fifteen males who smoked ‘large quantities’ of the substance and sixteen males who had not touched the drug. ‘Large quantities’ was defined by researchers as having smoked everyday for more than ten years and having smoked more than five ‘joints’ a day in this period. The researchers used magnetic resonance imaging to scan the brains of all subjects and made comparisons between the two subjects.

The results showed that all of the smoking participants in the study had a significantly smaller brain in these sections. The hippocampus was on average 12% smaller than the non-smoking sample, whilst the amygdala showed a seven percent decrease in size overall.

Murat Yucel, the leading scientist in the study stated: “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 test now needs to be carried out on larger numbers of people to find out if there is any credible link or if it was mere coincidence.

Similar studies were carried out in the U.S last month as a team at New York University scanned the brains of a group of 17 to 30 year olds who had smoked cannabis two or three times a week for a year or more. In that study, however, no significant changes in brain size were recorded.

The studies open fresh debates on the long term effects of the plant. In 2004, Cyril D’Souza, professor of psychiatry at the Yale University claimed that THC (the active ingredient in cannabis) causes a whole range of side effects including mild schizophrenia, increased anxiety, delusional behaviour and poor memory and attention spans.

This news came shortly after the UK downgraded the drug from a class B to a class C, also in 2004. The aim of the reclassification was to free up Police and give them more time to chase ‘real criminals’ but the move caused much confusion amongst the British public as nobody knew where they stood regarding consumption and possession of the drug.

Recent studies suggest that the high amounts of THC in skunk (the stronger and more potent form of the plant) make the drug more dangerous and is more likely to cause the side effects that Cyril D’Souza outlined in his studies. Because of this, the government are planning on reclassifying the drug back to its original class B status. This will undoubtedly cause more confusion amongst the British public. If the proposals prove a success, the law will change in early 2009.