Smoking cannabis alters the size and shape of two key brain regions involved in emotion and motivation, research has shown.
Even light use of the drug once or twice a week warps the brains of young adults, US scientists found.
Professor Hans Breiter, one of the researchers from Northwestern University in Chicago, says: "This study raises a strong challenge to the idea that casual marijuana use isn't associated with bad consequences."
The scientists analysed magnetic resonance imaging (MRI) brain scans of 20 young cannabis users aged 18 to 25 years and brain scans of 20 non-users.
They found major differences in two areas, the nucleus accumbens and amygdala. Both are linked to emotions and motivation, and also associated with addiction.
In each case, changes were seen that were directly related to how much cannabis was smoked.
The nucleus accumbens of cannabis users was unusually large, while the amygdala was deformed.
"Some of these people only used marijuana to get high once or twice a week," says Prof Breiter. "People think a little recreational use shouldn't cause a problem, if someone is doing OK with work or school. Our data directly says this is not the case."
Co-author Dr Anne Blood, from Harvard Medical School, says: "These are core, fundamental structures of the brain. They form the basis for how you assess positive and negative features about things in the environment and make decisions about them."
The drug users in the study smoked cannabis at least once a week but were not psychologically dependent on their habit.
The scientists, whose findings are reported in the Journal of Neuroscience, believe the effects are the result of users' brains adapting to low-level exposure to cannabis.
Previous research has shown that rats given the psychoactive compound in cannabis, THC, "rewire" their brains.
"It may be that we're seeing a type of drug learning in the brain," says study leader Dr Jodie Gilman, an instructor in psychology at Harvard Medical School. "We think when people are in the process of becoming addicted, their brains form these new connections."
Commenting on the findings, drug addiction expert Dr Carl Lupica, from the US National Institute on Drug Abuse, says: "This study suggests that even light to moderate recreational marijuana use can cause changes in brain anatomy.
"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."
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