Dabbling in cannabis as a youngster can have long-lasting effects on the brain, with scientists finding that smoking the drug once is enough to alter brain structure.
Teenangers who had smoked cannabis once or twice by the age of 14 had more grey matter in areas related to emotion and memory.
The finding is a concern, because teenage brains are supposed to be getting rid of surplus cells – and tests showed that the increased brain volume was linked to poor performance in IQ tests.
Researchers scanned the brains of 46 European young people, and found excess grey matter in the amygdala and hippocampus, linked to emotions and memory.
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.
‘The implication is that this is potentially a consequence of cannabis use. Most people would likely assume that one or two joints would have no impact on the brain.
‘One possibility is they’ve actually disrupted that pruning process.’
Prof Sir Robin Murray Professor of Psychiatric Research, King’s College London (IoPPN), said: ‘Previous studies of brain structure have been contradictory with some suggesting that heavy use is associated with decreased brain volumes, some no effect, and some the opposite.
‘However, it remains a small study and it is very surprising that persistent brain changes could result from the use of cannabis (or any other recreational drug) only once or twice.’