Smoking pot shrinks your brain, study says

Sydney  - Research in Australia has poked a hole in the common view that smoking marijuana is a harmless activity.

Years of heavy use of a substance derived from the cannabis plant can shrink important parts of the brain by 12 per cent, Melbourne University researchers said Tuesday.

Brain scans of 15 men who smoked at least five marijuana joints a day for at least 20 years showed their hippocampus was 12 per cent smaller and the amygdala was 7 per cent smaller than in a control group of 16 men who didn't smoke marijuana.

The hippocampus regulates memory and emotion while the amygdala plays a role in governing fear and aggression.

The 15 users also performed badly on in a memory test that required them to remember a list of 15 words.

"These findings challenge the widespread perception of cannabis as having limited or no consequences on the brain," clinical neuropsychologist Murat Yucel told the Herald Sun newspaper.

"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 study is published in the American Medical Association's journal Archives of General Psychiatry.

The 15 men studied had an average age of 39 and had not regularly taken other illicit drugs. (dpa)