One or two joints is all it takes to alter teenage brains, UK research shows
By John von Radowitz, Press Association Science Correspondent
Even a small amount of cannabis can alter brain structure in children, research suggests.
Scientists found that taking the drug just once or twice by the age of 14 led to greater "grey matter" volume in key brain areas linked to emotion and memory.
Grey matter mostly consists of nerve cells, while "white matter" is made up of nerve fibres.
The effects of the changes are unknown. But boosting nerve cell numbers may not be as beneficial as it sounds in developing brains, the US researchers point out.
At about the age of 14, the adolescent brain undergoes a form of "pruning" which removes unwanted neural connections, allowing the most important ones to be strengthened.
Lead scientist Professor Hugh Garavan, from the University of Vermont, said: "Consuming just one or two joints seems to change grey matter volumes in these young adolescents.
"One possibility is they've actually disrupted that pruning process."
The study is part of a long-term European project called Imagen that has collected brain-imaging data from 2,000 children and young adults.
Among the participants were 46 children from England , France and Germany who had reported smoking cannabis once or twice.
Compared with youngsters who had never smoked a joint, they had more grey matter in brain regions rich in cannabinoid receptors - places where the drug's chemicals bind to neurons.
Some of the biggest differences were seen in the amygdala and hippocampus, which play key roles in emotional processing and memory.
The team, whose findings are reported in the Journal of Neuroscience, ruled out the possibility that the cannabis-using children already had the unusual brain structure before experimenting with the drug.
"The implication is that this is potentially a consequence of cannabis use," said Prof Garavan. "You're 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."