Smoking cannabis just once as a teenager is enough to alter brain structure, a new study suggests.

Scientists found taking the drug just once or twice by the age of 14 led to greater grey matter in brain areas linked to emotion and memory.

Although more brain cells might seem beneficial, in fact the developing teenage brain at this stage should be pruning cells as it prepares for adulthood, removing unwanted neural connections and allowing important links to be strengthened.

In the study, greater brain volume was also associated with slightly worse IQ performance on tests.

Advertisement

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."

The study scanned the brains of 46 teenagers from England, France and Germany who had reported smoking once or twice, and compared them with the brains of youngsters who had never smoked a joint.

The excess of grey matter was found in areas of the brain which are known to be stimulated by cannabis, with the biggest differences found in the amygdala and hippocampus, which play a role in processing emotions and memory.

Sir Robin Murray, professor of psychiatric research, King's College London, said: "It remains a small study and it is very surprising that persistent brain changes could result from the use of cannabis only once or twice."

The research contradicts previous studies.

In 2017, two Dutch studies which followed young adults over a three-year period found no evidence of cannabis affecting brain structure, and a recent Australian study of 120 non-users and 140 users also did not detect any differences.

Professor David Nutt, of Imperial College London, said: "While this study alone is not able to prove small amounts of cannabis negatively affect the brains of adolescents, this area of research is important and certainly worthy of further study."

The research was reported in the Journal of Neuroscience.