Do you smoke marijuana? It turns out that even casual marijuana use may change your brain. Scientists have found that the size and shape of two brain regions involved in emotion and motivation may actually differ in young adults who smoke the substance at least once per week.

More and more states are legalizing marijuana for medical used. At the same time, though, teens use the drug recreationally. In fact, marijuana is the most commonly used illicit drug in the United States. It's estimated that about 18.9 million people reported recent use. Because of this, it's crucial to find out how the substance might affect people in the long term.

The main psychoactive component of marijuana is tetrahydrocannabinol (THC). Previous studies with animals have actually showed that repeated exposure to the drug causes structural changes in brain regions associated with motivation, attention, learning and memory. In order to find out a bit more, the scientists decided to take a look at how low exposures to marijuana might impact the brain.

The scientists used magnetic resonance imaging (MRI) to compare the brains of 18 to 25-year-olds who reported smoking marijuana at least once per week with the brains of those who had little or no history of marijuana use. In the end the scientists found that the nucleus accumbens, the part of the brain associated with reward processing, was larger and altered in its shape and structure in marijuana users.

"This study suggests that even light to moderate recreation marijuana use can cause changes in brain anatomy," said Carl Lupica, a scientist who studies drug addiction and who was not involved in the study, in a news release. "These observations are particularly interesting because previous studies have focused primarily on the brains of heavy marijuana smokers, and have largely ignored the brains of casual users."

The findings reveal that it's more important than ever to find out how marijuana can impact a person's brain. As the use of this drug spreads, it's crucial to find out what potential consequences are to using it.

The findings are published in the Journal of Neuroscience.