Using even a small amount of cannabis may cause changes in the brain volume of teenagers, according to a study.The study, published in The Journal of Neuroscience, is the first to find evidence that an increase in grey matter volume in certain parts of the adolescent brain is a likely consequence of low-level marijuana use.

Few studies have looked at the effects of the first few uses of a drug, said Hugh Garavan, a professor at the University of Vermont in the US.Most researchers focus on heavy marijuana users later in life and compare them against non-users. These new findings identify an important new area of focus.

Their brains showed more grey matter volume in areas where cannabis binds, known as cannabinoid receptors, compared to the children who did not use the drug.

The biggest differences in grey matter were in the amygdala, which is involved in fear and other emotion-related processes, and in the hippocampus, involved in memory development and spatial abilities.

The researchers ruled out the likelihood that the cannabis-using kids had pre-existing differences in grey matter thickness or that they had specific personality traits that might correlate with the difference in brain makeup.

“You are changing your brain with just one or two joints. Most people would likely assume that one or two joints would have no impact on the brain,” he said.