The Claim
Casual cannabis use harms young people's brains.

The Facts
A study found differences in the brains of users and nonusers, but it did not establish that marijuana use caused the variations or that they had any functional significance.

The Details
Researchers at Northwestern University and Harvard Medical School conducted MRI scans of two groups of 20 young adults ages 18 to 25. One group reported using marijuana at least once a week, smoking 11 joints a week on average, whereas the other had used it less than five times total and not at all during the last year. Neither group had any psychiatric disorders, and the users were psychiatrically assessed as not dependent on the drug.

The study focused on two brain regions involved in processing rewards, the nucleus accumbens and the amygdala. These areas create pleasurable experiences of things such as food and sex, as well as the high associated with drugs, and have been shown to change in animals given THC, the main psychoactive component of cannabis.

The researchers found that cannabis users had more gray matter density in the left nucleus accumbens and left amygdala, as well as differences in the shape of the left nucleus accumbens and right amygdala. The left nucleus accumbens also tended to be slightly larger in users. They concluded that recreational cannabis use might be associated with abnormalities in the brain's reward system. News reports have proclaimed that scientists have shown that even casual cannabis use harms young people's brains.

The Caveats
The most obvious problem with leaping to that conclusion is that the scans were conducted at only one point. This approach can compare the two groups, but it cannot prove cannabis caused any differences between them—or even that the differences represent changes over time. They could be preexisting variations, or cannabis use and brain changes may both be related to a third factor, such as tobacco (although the study did attempt to take levels of smoking into account).

That said, it is plausible that the dissimilarities were a result of using cannabis. By definition, all psychoactive substances cause changes in the brain. Recreational drugs such as cannabis stimulate our reward system, triggering the nucleus accumbens to release dopamine and generate an experience of pleasure—which is why people take them. Increasing dopamine activity will subtly alter the brain, but even playing the lottery on a regular basis might produce such a change. “I think that's all we're seeing here,” says Robin Murray, professor of psychiatry at King's College London. “It's likely these are adaptive changes, which will probably disappear when they stop taking cannabis.”

More important, the scientists did not measure cognitive performance, and they found no links between their data and the occurrence of mental health problems. Calling the finding “damage” is therefore arbitrary. “These differences were not associated with any problems,” says Tom Freeman, a researcher at University College London. “So further evidence is needed to conclude [the differences] are harmful in any way.”

Reality Check—Cannabis use has been found to:

• Cause dependence, at some point in their lives, in about 9 percent of people who try it.

• Impair various aspects of cognitive function, particularly memory. Impairments can remain for several days. One study showed that performance returns to nonusers' levels after 28 days of abstinence, but evidence is mixed about how long the impairments last.

• Potentially reduce the volume of the hippocampus, which is critical for memory—but only after heavy and prolonged use. The evidence linking cognitive impairments to specific brain changes is inconclusive, and the degree to which such changes are reversible is hotly debated. —S.M.