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Smoking pot just once might be linked to changes in young brains, a new study suggests.

Adolescents who smoked marijuana one or two times had increased amounts of gray matter — a combination of neuron bodies and cells that feed them — in their brains compared with those who hadn’t, according to a new study published yesterday (Jan. 14) in the Journal of Neuroscience.

A bigger brain volume may sound like a benefit for adolescents, but it may not be, said lead study author Catherine Orr, a lecturer in the department of psychological sciences at Swinburne University of Technology in Australia. [7 Ways Marijuana May Affect the Brain]

That’s because, in adolescence, the brain typically undergoes a process called pruning, Orr said. During this process, which goes on through a person’s early 20s, the brain chips away at its existing gray matter, and increases the amount of white matter, which consists of the long tails of neurons called axons that connect cells, and their protective coating called myelin.

Though it’s not absolutely clear why the brain does this, the best explanation to date is that the process makes the brain more efficient, cutting away unnecessary or redundant components, and also makes the brain more complex by building more connections, Orr said.

That’s why interfering in the process — by adding more gray matter — could potentially be a bad thing.

In their study, Orr and her team looked at brain scans from 46 teens, all age 14, from Ireland, France, England and Germany. Some reported having smoked marijuana once or twice while others reported never having used the drug at all. The researchers controlled for other factors such as cigarette smoking and alcohol-use that could have also driven these changes.

The study found that those who smoked pot had higher volumes of gray matter in areas of the brain that had cannabinoid receptors, compared with those who reported never using the drug. Compounds in marijuana bond to cannabinoid receptors, which are found in areas including the amygdala (which is involved in emotion and threat processing), the hippocampus (which is involved in memory and learning), and the nucleus accumbens (which is involved in reward and addiction).

The researchers also found that those who had increased gray matter in their brains also had lower perceptual reasoning and a lower ability to perform a task quickly than those with normal levels of gray matter

“I personally was surprised at how extensive the effects were,” Orr told Live Science.

Previous studies had found marijuana affects similar areas of the brain, however, these studies have had inconsistent results. For example, some studies in adults have found that marijuana-use was linked to an increase in gray matter, while others found the opposite. Orr noted that because these earlier mostly looked at adult brains, it’s difficult to untangle the potential effects of other substances such as alcohol.

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