*The following is excerpted from an online article posted on HealthDay.
Smoking just a couple of joints may cause significant changes in a teenager's brain structure, a new study has found.
Brain scans show that some adolescents who've tried marijuana just a couple of times exhibit significant increases in the volume of their gray matter.
These changes were associated with increased risk of anxiety, and decreased ability on thinking and memory tests.
"It is important to understand why some people may be more vulnerable to brain effects of cannabis at even the earliest stages of use, as it might give us some insight into why some people transition to substance misuse while others do not," said lead researcher Catherine Orr. She is a lecturer at Swinburne University of Technology in Melbourne, Australia.
"Also, if we can identify some of the factors that place people at greater risk of these brain effects, we need to let people know what they are so that they can make informed decisions about their substance use," Orr continued.
Most studies involving the effects of pot on the brain focus on heavy marijuana users. These researchers wanted to focus instead on what might happen as teens experiment with marijuana.
To that end, they gathered brain scan data obtained as part of a large research program investigating brain development and mental health in teens.
The researchers examined brain imaging of 46 kids, aged 14 years, from Ireland, England, France and Germany, who reported trying pot once or twice. They also looked at the teens' scores on cognitive and mental health tests.
The teens' brains showed greater gray matter volume in brain areas more affected by pot, when compared with kids who'd never toked, the study authors said.
"The regions of the brain that showed the volume effects map onto the parts of the brain that are rich in cannabinoid receptors, suggesting that the effects we observe may be a result of these receptors being stimulated by cannabis exposure," Orr said.
Regions most affected by weed were the amygdala, which is involved in processing fear and other emotions, and the hippocampus, which is involved with memory and reasoning, the researchers said.
The findings were published in the Journal of Neuroscience.