What the increased brain matter volume means is unclear.

"The implication is that this is potentially a outcome of cannabis use", commented senior study author Hugh Garavan, PhD, a University of Vermont (UVM) professor of psychiatry. She is a lecturer at Swinburne University of Technology in Melbourne, Australia.

Dr Orr's research has also shown there is a range of effects within the group of cannabis-using teens, which could be related to cognition and emotion.

"In our sample of cannabis users, the greater volumes in the affected parts of the brain were associated with reductions in psychomotor speed and perceptual reasoning and with increased levels of anxiety two years later", Orr said.

It would explain why gray matter in the cannabis users was thicker than the gray matter in the non-users. "Therefore, these findings ought to be regarded with caution". The researchers analyzed images from 46 14-year-olds who said they had used marijuana once or twice, as well as images from 46 non-cannabis using teens matched "on age, sex, handedness, pubertal status, IQ, socioeconomic status, and use of alcohol and tobacco", Orr said.

To try to address this question, the researchers analyzed scans from a third group of teens who had not tried marijuana before they had their brain scans at age 14. They also looked at the teens' scores on cognitive and mental health tests.

"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. The most noticeable changes were in areas called the amygdala (associated with fear and other emotion-related processes) and the hippocampus (associated with memory development and spatial abilities). The study was published in The Journal of Neuroscience.

Fourteen-year-olds who say they have smoked just one or two joints of marijuana in their lifetime demonstrate structural changes to multiple regions of their brains, according to the results of research by an worldwide team of scientists across Europe and North America. Garavan is a professor of psychiatry with the University of Vermont.

'Most people would likely assume that one or two joints would have no impact on the brain'. There may be serious implications to the brain changes noted by the researchers. There are a lot of things that influence brain development in teens that can't be ruled out by the data at hand. Rates of cannabis use among adolescents are now high.

Whilst acknowledging that the study does have a number of limitations, they further suggested that the findings are consistent with previous reports of a dose-response effect of cannabis on behavioral and brain measures following heavier use.