Marijuana doesn't appear to shrink important regions of the brains of users, but a study published Wednesday shows something possibly more subtle and important: the brains of people who tend to use marijuana may be smaller to start with.
A second study found that marijuana appears to change the brain structure of young men with a high genetic risk of schizophrenia.
The studies, both published in the Journal of the American Medical Association's JAMA Psychiatry, don't paint a clear picture of anything, says Dr. David Goldman of the National Institute on Alcohol Abuse and Alcoholism.
"It's probably more a story of what we don't know than what we do know," Goldman, who wrote a commentary on the reports, told NBC News. It does not mean that cannabis is safe, he stressed.
In the first study, David Pagliaccio, formerly of Washington University in St. Louis and now at the National Institute of Mental Health, and colleagues used magnetic resonance imaging to compare the brains of marijuana users to non-users.
They included batches of siblings, one who used marijuana and the other who didn't, to try to tease out whether brain differences might be genetic instead of being linked to marijuana use.
When they looked at all 482 volunteers, it did seem that the marijuana users had some shrinkage in two brain regions called the amygdala and the right ventral striatum. But when they compared marijuana users to their siblings, the differences disappeared. So it's possible that people who choose to use marijuana may already have the smaller regions.
It's not clear why, Goldman said. "It could be nutrition, it could be stress exposures, it could be a lot of different things," he said.
There were other differences, Goldman said. "The cannabis-exposed individuals were worse off," he wrote. "They tended to be poorer, less agreeable, more likely to use other drugs, and more likely to discount larger future rewards for the immediacy of smaller ones."
But whatever cannabis is doing to the brain, it's not evident on this particular type of MRI scan, he said.
"There is something else that is going on," Goldman said. THC, one of the main active components of cannabis, is a psychoactive drug. These types of drugs affect brain function, if not its physical structure. "They alter the connectivity of regions of the brain," he said. "And they alter the function of particular neurons and circuits."
As with most medical studies, these two add pieces to a larger puzzle, Goldman said. None can be expected to be the final word.
"Restrictions on cannabis use are being loosened although we do not know enough about cannabis' effects," Goldman added in an email.
"These new studies show that some effects of cannabis use on brain volumes, although a big concern, are probably innate, rather than due to the drug exposure. However, there are many other effects of cannabis to be concerned with, including effects on the neurocircuit that can be seen in other ways," he said.
"We do know that more people will use cannabis heavily, and will access cannabis with much higher concentrations of THC, the active ingredient."
Other studies have strongly suggested that cannabis affects brain structure.
One study last year compared MRI images from the brains of 20 recreational weed smokers (who puff a mean of 11 joints per week) and the brains of 20 non-users. They found abnormalities in the brains of the users.
A 2013 study found the brains of young, heavy marijuana users were altered in sub-cortical regions —part of the memory and reasoning circuitry.
In the second study released Wednesday, Dr. Tomáš Paus of the Rotman Research Institute in Toronto and colleagues interviewed 1,500 teenagers at high genetic risk of schizophrenia.
The boys who admitted they used marijuana had thinner brain cortices, they found. The cortex is the folded outer layer of the brain that carries much of the brain's gray matter.
"Our findings suggest that cannabis use might interfere with the maturation of the cerebral cortex in male adolescents at high risk for schizophrenia," they wrote.
Again, it's not completely clear what that might mean for the pot-smokers, the experts said. It doesn't show that smoking dope raises the risk of schizophrenia, but it does support the idea that some people are more susceptible than other to damaging effects from marijuana.
And it adds to a body of studies showing that marijuana use in the teen years, when the brain is still developing, can alter that development.