The study to be published on Wednesday in the Journal of Neuroscience differs from many other pot-related research projects that are focused on chronic, heavy users of cannabis.
The collaborative effort between Northwestern University's medical school, Massachusetts General Hospital and Harvard Medical School showed a direct correlation between the number of times users smoked and abnormalities in the brain.
"What we're seeing is changes in people who are 18 to 25 in core brain regions that you never, ever want to fool around with," said co-senior study author Dr. Hans Beiter, professor of psychiatry and behavioral sciences at Northwestern University.
In particular, the study identified changes to the nucleus accumbens and the nucleus amygdala, regions of the brain that are key to regulating emotion and motivation, in marijuana users who smoke between one and seven joints a week.
The researchers found changes to the volume, shape and density of those brain regions. But more studies are needed to determine how those changes may have long-term consequences and whether they can be fixed with abstinence, Beiter said.
"Our hypothesis from this early work is that these changes may be an early sign of what later becomes amotivation, where people aren't focused on their goals," he said.
The study, which was funded in part by the National Institute on Drug Abuse and the White House Office of National Drug Control Policy, comes as access to pot is expanding following 2012 votes in Washington state and Colorado to legalize its recreational use. The drug remains illegal under federal law.
Medical pot is allowed in 20 U.S. States.
Pot legalization advocates make the argument that marijuana is safer than alcohol a central part of their campaigns.
Other research has found drinking alcohol alters the brain, Beiter said. But while researchers do not know exactly how the mental rewiring seen in pot users affects their lives, the study shows it physically changes the brain in ways that differ from drinking, he said.
This latest study fits with other research showing marijuana use has significant effects on young people because their brains are still developing, and Beiter said he has become convinced that marijuana should only be used by people under 30 if they need it to manage pain from a terminal illness.