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

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."