Marijuana can mess with your memory by changing the way your brain processes information, but scientists still aren’t sure exactly how this happens. Still, several studies suggest that weed interferes with short-term memory, and researchers tend to see more of these effects in inexperienced or infrequent users than in heavy, frequent users. Unsurprisingly, these effects are most evident in the acute sense — immediately after use, when people are high.
According to the new NASEM report, there was limited evidence showing a connection between cannabis use and impaired academic achievement, something that has been shown to be especially true for people who begin smoking regularly during adolescence. (That has also been shown to increase the risk for problematic use.)
Importantly, in most cases, saying cannabis is connected to an increased risk doesn’t mean marijuana use caused that risk.
And in some people, weed could increase the risk of depression …
Scientists can’t say for sure whether marijuana causes depression or depressed people are simply more likely to smoke. But one study from the Netherlands suggests that smoking weed could raise the risk of depression for young people who already have a special serotonin gene that could make them more vulnerable to depression.
Those findings are bolstered by the NASEM report, which found moderate evidence that cannabis use was linked to a small increased risk of depression.
… and it may also increase the risk of developing schizophrenia.
The NASEM report also found substantial evidence of an increased risk among frequent marijuana users of developing schizophrenia — something that studies have shown is a particular concern for people at risk for schizophrenia in the first place.
Regular marijuana use may also be connected to an increased risk of social anxiety.
Researchers think it’s possible that CBD might be a useful treatment for anxiety disorders, and that’s something that several institutions are currently trying to study.
And in general, the recent report thought the evidence that marijuana increased the risk of most anxiety disorders was limited.
However, the authors write that there is moderate evidence that regular marijuana use is connected to an increased risk of social anxiety. As in other cases, it’s hard to know whether marijuana use causes that increase or people use marijuana because of an increased risk of social anxiety.
Most importantly, regular weed use is linked with some specific brain changes — but scientists can’t say for sure whether one causes the other.
In a recent study, scientists used a combination of MRI brain scans to get a better picture of the brains of adults who have smoked weed at least four times a week for years.
Compared with people who rarely or never used, the long-term users tended to have a smaller orbitofrontal cortex, a brain region critical for processing emotions and making decisions. But they also had stronger cross-brain connections, which scientists think smokers may develop to compensate.
Still, the study doesn’t show that smoking pot caused certain regions of the brain to shrink; other studies suggest that having a smaller orbitofrontal cortex in the first place could make someone more likely to start smoking.
Most researchers agree that the people most susceptible to brain changes are those who begin using marijuana regularly during adolescence.
Marijuana use affects the lungs but doesn’t seem to increase the risk of lung cancer.
People who smoke marijuana regularly are more likely to experience chronic bronchitis, according to the report. There’s also evidence that stopping smoking relieves these symptoms.
Yet perhaps surprisingly, the report’s authors found moderate evidence that cannabis was not connected to any increased risk of the lung cancers or head and neck cancers associated with smoking.
Some think marijuana could be used in ways that might improve certain types of athletic performance.
Some athletes, especially in endurance and certain adventure sports, say marijuana use can boost their athletic performance. This may be because of anti-inflammatory or pain-relieving effects that make it easier to push through a long workout or recover from one.
At the same time, there are ways that marijuana could impair athletic performance by affecting coordination and motivation or by dulling the body’s natural recovery process.
Without more research, it’s hard to know for sure how marijuana affects athletic performance.
There’s evidence that marijuana use during pregnancy could have negative effects.
According to the new NASEM report, there’s substantial evidence showing a link between prenatal cannabis exposure — when a pregnant woman uses marijuana — and lower birth weight. There was limited evidence suggesting that this use could cause pregnancy complications and increase the risk that a baby would have to spend time in a neonatal intensive care unit.
There are still so many questions about how marijuana affects the body and brain that scientists say far more research is needed.
Based on the report and conversations with researchers, there are good reasons to think marijuana has potentially valuable medical uses. At the same time, we know that, as with any substance, not all use is risk-free.
More research is needed to figure out how to best treat the conditions that cannabis can help and how to minimize any risks associated with medical or recreational use.
That research is essential so that we know “how best we can use it, what are the safest ways, and what are the real risks,” Staci Gruber, an associate professor of psychiatry at Harvard Medical School and director of the Marijuana Investigations for Neuroscientific Discovery program at McLean Hospital, told Business Insider.