A study published in the Journal of Neuroscience may be the first link to show that casual pot use may cause major changes to the brain.

Using different types of neuroimaging, researchers examined 40 brains of people between the ages of 18 and 25. 20 of them smoked marijuana at least once a week, the other 20 did not use pot at all.

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The result had Dr. Hans Breiter, co-senior author of the study and a professor of psychiatry and behavioral sciences at Northwestern University Feinberg School of Medicine wary about young adults using pot.

“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.”

According to the National Survey on Drug Use and Health, in 2012 nearly 19 million Americans used pot making it the most illegally used drug in the world.

See the full study here.