Substantial evidence from animal research and a growing number of studies in humans indicate that marijuana exposure during development can cause long-term or possibly permanent adverse changes in the brain. Rats exposed to THC before birth, soon after birth, or during adolescence show notable problems with specific learning and memory tasks later in life. 32–34 Cognitive impairments in adult rats exposed to THC during adolescence are associated with structural and functional changes in the hippocampus. 35–37 Studies in rats also show that adolescent exposure to THC is associated with an altered reward system, increasing the likelihood that an animal will self-administer other drugs (e.g., heroin) when given an opportunity (see “Is marijuana a gateway drug?”).
Marijuana, Memory, and the Hippocampus
A large longitudinal study in New Zealand found that persistent marijuana use disorder with frequent use starting in adolescence was associated with a loss of an average of 6 or up to 8 IQ points measured in mid-adulthood. 43 Those who used marijuana heavily as teenagers and quit using as adults did not recover the lost IQ points. People who only began using marijuana heavily in adulthood did not lose IQ points. Two shorter-duration prospective longitudinal twin studies found that youth who used marijuana showed significant declines in verbal ability (equivalent to 4 IQ points) and general knowledge between the preteen years (ages 9 to 12, before use) and late adolescence/early adulthood (ages 17 to 20); however those who went on to use marijuana at older ages already had lower scores on these measures at the start of the study, before they started using the drug. Also, no predictable difference was found between twins when one used marijuana and one did not. 44
More research will be needed to answer definitively whether marijuana use causes long-term IQ losses and whether factors that weren’t measured in the prior research, such as the increasing amounts of THC in cannabis and the emergence of new cannabis products, are relevant.
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.
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.
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.
“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.
“It’s probably more a story of what we don’t know than what we do know."
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.
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.
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.