Many marijuana users contend that smoking pot in moderation and occasionally isn't harmful, but a new study by researchers from the Northwestern University medical school, Harvard Medical School and Massachusetts General Hospital found evidence that smoking marijuana, even in moderation, can still lead to changes in the brain, particularly in young users.

In a study published in the Journal of Neuroscience Wednesday, April 16, researchers involved 20 marijuana users between 18-25 years old who said they smoke pot recreationally. The participants smoke an average of about 11 joints a week, with half of them smoking fewer than six joints per week.

The researchers scanned the brains of these occasional pot users and then compared the results with those of 20 other participants who do not smoke marijuana but shared similar traits such as sex and age with the pot users.

Comparing the results of the brain scans, the researchers found differences in the volume, shape and density of the nucleus accumbens and the nucleus amygdala, the parts of the brain that are associated with emotion and motivation.

The differences were notably more evident in subjects who claimed to smoke more marijuana. The nucleus accumbens of marijuana users were larger than those of nonusers, but the changes are directly proportional to the number of joints that the person used.

"This study suggests that even light to moderate recreational marijuana use can cause changes in brain anatomy," said Carl Lupica from the National Institute on Drug Abuse (NIDA). "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." He studies drug addiction and was not involved in the research study.

Study researcher Hans Beiter, from Northwestern University's Feinberg School of Medicine, and colleagues did not look at whether the changes in the brain structure of marijuana users were associated with a decline in brain function. Nonetheless, they said that these may suggest that young people are particularly vulnerable to the effects of marijuana because their brains have not yet fully developed.

Although there are already a number of research studies that examine the effects of marijuana, the focus often is on chronic and heavy pot users. The new study, which was partly funded by the White House Office of National Drug Control Policy and the NIDA, focused on young and casual users.