Marijuana's official designation as a Schedule 1 drug — something with "no currently accepted medical use" — means it's pretty tough to study.
Yet both a growing body of research and numerous anecdotal reports link cannabis with several health benefits, including pain relief and helping with certain forms of epilepsy. In addition, researchers say there are many other ways marijuana might affect health that they want to better understand.
A massive new report released in January by the National Academies of Sciences, Engineering, and Medicine helps sum up exactly what we know — and, perhaps more important, what we don't know — about the science of weed.
Marijuana can make you feel good.
One of weed's active ingredients, tetrahydrocannabinol, or THC, interacts with our brain's reward system, the part that has been primed to respond to things that make us feel good, like eating and sex.
When overexcited by drugs, the reward system creates feelings of euphoria. This is also why some studies have suggested that excessive marijuana use can be a problem in some people — the more often you trigger that euphoria, the less you may feel during other rewarding experiences.
In the short term, it can also make your heart race.
Within a few minutes of inhaling marijuana, your heart rate can increase by between 20 and 50 beats a minute. This can last anywhere from 20 minutes to three hours, according to the National Institute on Drug Abuse.
The new report found insufficient evidence to support or refute the idea that cannabis might increase the overall risk of a heart attack. The same report, however, also found some limited evidence that smoking could be a trigger for a heart attack.
Weed may also help relieve some types of pain ...
Pot also contains cannabidiol, or CBD — and this chemical, while not responsible for getting you high, is thought to be responsible for many of marijuana's therapeutic effects such as pain relief or potentially treating certain kinds of childhood epilepsy.
The new report also found conclusive or substantial evidence — the most definitive levels — that cannabis can be an effective treatment for chronic pain, which could have to do with both CBD and THC. Pain is also "by far the most common" reason people request medical marijuana, according to the report.
... like the discomfort of arthritis ...
One of the ways scientists think it may help with pain is by reducing inflammation, a component of painful illnesses like rheumatoid arthritis.
A preliminary 2005 study of 58 patients with RA, roughly half of whom were given a placebo and roughly half of whom were given a cannabis-based medicine called Sativex, found "statistically significant improvements in pain on movement, pain at rest, quality of sleep" for patients on Sativex.
Other studies testing both other cannabinoid products and inhaled marijuana have shown similar pain-relieving effects, according to the report.
... or the pain of inflammatory bowel disease.
Some people with inflammatory bowel diseases like Crohn's and ulcerative colitis could also benefit from marijuana use, studies suggest.
A 2014 paper, for example, describes two studies of people with chronic Crohn's in which half were given the drug and half were given a placebo. That study showed a decrease in symptoms in 10 of 11 subjects using cannabis, compared with just four of 10 on the placebo. But when the researchers did a follow-up study using low-dose CBD, they saw no effect in the patients.
Researchers say that, for now, we need more research before we'll know whether cannabis can help with these diseases.
Marijuana may also be helpful in controlling epileptic seizures.
A drug called Epidiolex, which contains CBD, may be on its way to becoming the first of its kind to win approval from the Food and Drug Administration for the treatment of rare forms of childhood epilepsy.
The company that makes it, GW Pharma, is exploring CBD for its potential use in people with Dravet syndrome, a rare form of childhood-onset epilepsy that is associated with multiple types of seizures.
In March, the company came out with phase three trial data that showed the drug had some positive results.
But it can also mess with your sense of balance.
It may throw off your balance, as it influences activity in the cerebellum and basal ganglia, two brain areas that help regulate balance, coordination, reaction time, and posture.
And it can distort your sense of time.
Feeling as if time is sped up or slowed down is one of the most commonly reported effects of using marijuana. A 2012 paper sought to draw some more solid conclusions from some of the studies on those anecdotal reports, but it was unable to do so.
"Even though 70% of time estimation studies report overestimation, the findings of time production and time reproduction studies remain inconclusive," the paper said.
In a 1998 study that used magnetic resonance imaging to focus on the brains of volunteers on THC, the authors noted that many had altered blood flow to the cerebellum, which most likely plays a role in our sense of time.
Limitations on what sort of marijuana research is allowed make it particularly difficult to study this sort of effect.
Weed can also turn your eyes red.
Since weed makes blood vessels expand, it can give you red eyes.
And you'll probably get the munchies.
A case of the munchies is no figment of the imagination — both casual and heavy marijuana users tend to overeat when they smoke.
Marijuana may effectively flip a circuit in the brain that is normally responsible for quelling the appetite, triggering us to eat instead, according to a recent study of mice.
It all comes down to a special group of cells in the brain that are normally activated after we have eaten a big meal to tell us we've had enough. The psychoactive ingredient in weed appears to activate just one component of those appetite-suppressing cells, making us feel hungry rather than satisfied.
Marijuana may also interfere with how you form memories.
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.
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