It is often described as one of the safest drugs, but a new study suggests that cannabis may not be as safe as thought.
A new study by researchers at the University of Vermont found that cannabis smoking is all it takes to change teenage brain.
The team found that smoking at the age of 14 years causes a greater amount of 'gray matter' in a number of key brain areas associated with memory and emotions.
And worrying, the impact of the changes is still unknown.
At the age of about 14, "pruning" takes place in the brain of the adolescent, preventing unwanted nervous connections, making it more important.
Leading scientist Professor Hugh Garavan from the University of Vermont said: "Only one or two joints in these young people change the amount of gray matter.
"One possibility they have actually stopped this pruning process."
The study is part of a long-term European project called Imagen, which has collected brain imaging data from 2000 children and young people.
Among the participants were 46 children from England, France and Germany who reported smoking one or two times.
Compared to youngsters who have never smoked, they had more gray matter in brain regions with many cannabis receptors – where drug chemicals bind to neurons.
Some of the biggest differences were in amygdala and hippocampus, which play the main roles in emotional processing and memory.
The team, whose findings were reported in the Neuroscience magazine, ruled out that the use of cannabis in children had an unusual brain structure before experimenting with the drug.
"This means that it is a potential result of cannabis use," said Prof Garavan. "You change your brain with one or two joints.
"Most people are likely to assume that one or two joints will not affect the brain."