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So. You know that occasional joint? Turns out, a new study says it might not be as okay as you thought it was.

In the first study to link casual marijuana use to major brain changes, researchers say that even occasionally smoking marijuana may rewire your brain and cause structural changes. The study is published in the Journal of Neuroscience.

Previous research has already revealed similar changes in brain structure among heavy marijuana users. But this is the first study to show that casual use can alter a person’s brain, according to study lead author Dr. Jodi Gilman, lead author and a researcher in the Massachusetts General Center for Addiction Medicine.

“We were interested in looking at adults who aren’t addicted,” Gilman said.

Using different types of neuroimaging, researchers examined the brains of 40 young adults between the ages of 18 and 25 who were enrolled in Boston-area colleges. Twenty of them smoked marijuana at least once a week. The other 20 did not use pot at all. None of the users reported any problems with school, work, legal issues, parents or relationships, according to Dr. Hans Breiter, co-senior author of the study and a professor of psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine.

Researchers examined regions of the brain involved in emotional processing, motivation and reward, called the nucleus accumbens and the amygdala. They analyzed volume, shape and density of grey matter – where most cells in brain tissue are located.

“There’s a general idea out there that casual use of marijuana does not lead to bad effects, so we started out to investigate that very directly,” Breiter said. “This research with the other studies we have done have led me to be extremely concerned about the effects of marijuana in adolescents and young adults and to consider that we may need to be very careful about legalization policies and possibly consider how to prevent anyone under age 25 to 30 from using marijuana at all.”

There has been an ongoing interest in studying the effects of marijuana, and other research has shown that early-onset pot smokers are slower at tasks, have lower IQs later in life and have increased stroke risks. Consequently, Gilman says that the next step in their research will be to see how these structural abnormalities relate to a pot smoker’s behavior.

“We think that abnormal neuronal growth is evidence that the brain is forming new pathways that could encourage future use of the drug,” she said. “We do know there are clinically observable behavioral differences in people who smoke marijuana heavily. Maybe some of these brain changes can relate to some of the behavioral changes that have been observed clinically.”

Breiter added that the pot being smoked by young people today is much more powerful than marijuana available to people in the 1960s. Today’s marijuana contains much greater concentrations of THC, the primary psychoactive ingredient in pot.

“Levels of THC are about sevenfold what they used to be,” he said. “That’s a substantial change in the dosing of THC that these young people are getting. The experience of people in the ’60s and ’70s may not be the same experience as people today.”

Experts are unsure whether findings such as these could have any impact on states that are considering following the lead of Colorado and Washington regarding legalizing marijuana.