Using even a small amount of cannabis may cause changes in the brain volume of teenagers, according to a study.
The study, published in The Journal of Neuroscience, is the first to find evidence that an increase in grey matter volume in certain parts of the adolescent brain is a likely consequence of low-level marijuana use.
Few studies have looked at the effects of the first few uses of a drug, said Hugh Garavan, a professor at the University of Vermont (UVM) in the US.
Most researchers focus on heavy marijuana users later in life and compare them against non-users. These new findings identify an important new area of focus.
"Consuming just one or two joints seems to change grey matter volumes in these young adolescents," Garavan said.
The study included 46 children who reported having used cannabis once or twice by age 14.
Their brains showed more grey matter volume in areas where cannabis binds, known as cannabinoid receptors, compared to the children who did not use the drug.
The biggest differences in grey matter were in the amygdala, which is involved in fear and other emotion-related processes, and in the hippocampus, involved in memory development and spatial abilities.
The researchers ruled out the likelihood that the cannabis-using kids had pre-existing differences in grey matter thickness or that they had specific personality traits that might correlate with the difference in brain makeup.
"The implication is that this is potentially a consequence of cannabis use," Garavan said.
"You are 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," he said.
However, the researchers said that what the increased brain matter volume means is unclear.
Typically at that age, Garavan said, the adolescent brain undergoes a "pruning" process, where it gets thinner, rather than thicker as it refines its synaptic connections.
"One possibility is they have actually disrupted that pruning process," Garavan added.