Darrin Harris Frisby / Drug Policy Alliance

Marijuana's official designation in the US as a Schedule 1 drug - something with "no currently accepted medical use" - means it has been pretty tough to study.

Despite that, a growing body of research and numerous anecdotal reports link cannabis with several health benefits, including pain relief and the potential to help with certain forms of epilepsy. In addition, researchers say there are many other ways marijuana might affect health that they want to better understand - including a mysterious syndrome that appears to make marijuana users violently ill.

Along with several other recent studies, a massive report released by the National Academies of Sciences, Engineering, and Medicine in 2017 helps sum up exactly what we know - and what we don't - about the science of weed. Here's what you should know about how marijuana affects the brain and body.


Marijuana use is linked to a rare syndrome that causes nausea and vomiting.

Getty Images/John Moore

Most recently, a March 2019 study looked at over 2,500 cannabis-related ER visits in Colorado. They found that stomach issues like nausea and vomiting were the main driver of the trips, even before psychiatric problems like intoxication and paranoia.

In 2004, Australian doctors began looking into these stomach symptoms based on the experiences of a local woman who used to be able to smoke marijuana with no issue, and then seemingly out of nowhere began having adverse reactions that paralleled those in the 2019 study.

They gave her condition a name: cannabinoid hyperemesis syndrome, or CHS. The rare illness is still fairly new and understudied, but researchers believe it might affect a large population.

"CHS is certainly not very rare," Andrew Monte, an associate professor of emergency medicine at UCHealth University of Colorado Hospital who led the March study, previously told Business Insider. "We see it absolutely every week in our ER."


Marijuana can make you feel good.

One of weed's active ingredients, tetrahydrocannabinol (THC) interacts with the 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 for 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 NASEM 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.


Marijuana's effects on the heart could be tied to effects on blood pressure, but the link needs more research.

In August, a study published in the European Journal of Preventive Cardiology appeared to suggest that marijuana smokers face a threefold higher risk of dying from high blood pressure than people who have never smoked - but the study came with an important caveat: it defined a "marijuana user" as anyone who'd ever tried the drug.

Research suggests this is a poor assumption - and one that could have interfered with the study's results. According to a recent survey, about 52% of Americans have tried cannabis at some point, yet only 14% used the drug at least once a month.

Other studies have also come to the opposite conclusion of the present study. According to the Mayo Clinic, using cannabis could result in decreased - not increased - blood pressure.

So while there's probably a link between smoking marijuana and high blood pressure, there's not enough research yet to say that one leads to the other.


Weed may also help relieve some types of pain.

Pot contains cannabidiol, or CBD, a chemical that is not responsible for getting you high but is thought to be responsible for many of marijuana's therapeutic effects. Those benefits can include pain relief or potential treatment for 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.


Pain relief could include the discomfort of arthritis.

Robert Johnson for Business Insider

One of the ways scientists think marijuana may help with pain is by reducing inflammation, a component of 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 other cannabinoid products and inhaled marijuana have shown similar pain-relieving effects, according to the report.


Individuals with inflammatory bowel disease may also find some relief.

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. Half were given the drug and half got 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.

Sonya Yruel/Drug Policy Alliance

In June 2018, the FDA approved a CBD medication called Epidiolex.

The drug can be prescribed to people with Lennox-Gastaut syndrome and Dravet syndrome, two rare forms of epilepsy. In fact, it is the first FDA-approved treatment option for Dravet syndrome.

In the clinical trial for the drug, common side effects included sleepiness, fatigue, decreased appetite, diarrhea, and insomnia.


But it can also mess with your sense of balance.

Marijuana 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 solid conclusions from 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 (MRI) 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.

A recent study in mice suggested the possibility that marijuana may effectively flip a circuit in the brain that is normally responsible for quelling the appetite, triggering us to eat instead.

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.

A 2014 study found that marijuana use had no effect on body weight, despite the munchies phenomenon commonly associated with use.


Some women have reported having more satisfying sex when using marijuana.

A small study of 373 women from of varying races, sexual orientations, and marital statuses found that people who said they used marijuana before sex tended to have a more pleasurable experience than those who did not use the substance.

Specifically, some women reported having more satisfying orgasms and an increase in their sex drive.

Researchers weren't able to pinpoint why marijuana had this effect, but suggested it could be due to the substance's ability to reduce stress and anxiety.


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.


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.

The recent report suggested that evidence of a link between marijuana and an increased risk of most anxiety disorders was limited.

However, the authors wrote 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.


Marijuana use could also affect sperm count.

A small study of 37 men found that those who used marijuana had decreased sperm counts than those who never used marijuana. The study did not specify the methods of marijuana consumption used.

Another study, however, found that marijuana increased sperm count in men. This study was larger and looked at 1,215 healthy young men. They found that men who currently or previously used marijuana had both higher sperm counts and higher sperm concentrations than men who never used the substance.


A person could need more sedation medication for surgery if they have a regular marijuana habit.

A small study in the May 2019 issue of The Journal of the American Osteopathic Association found that people who reported smoking marijuana or eating edibles on a daily or weekly basis needed higher doses of medication for sedation.

The researchers found that people who used marijuana daily or weekly needed 14% more fentanyl, 19.6% more midazolam, and 220.5% more propofol (all medications that are used for sedation) for the full length of their procedures than their counterparts who didn't use as much marijuana.

Researchers haven't determined why this was the case, but they believe marijuana could potentially desensitize the body's receptors that process sedatives.


It's worth noting that marijuana has likely gotten stronger since the 1980s, and that its ratio of THC to CBD has changed as well.

The THC content of marijuana across the US has tripled since 1995, according to a large recent study in which researchers reviewed close to 39,000 samples of cannabis. While THC levels hovered around 4%, on average, in 1995, they skyrocketed to roughly 12% in 2014.

Meanwhile, the CBD content in marijuana - the part that's responsible for many of the drug's therapeutic effects - has dropped, the researchers found, shifting the ratio of THC to CBD from 14:1 in 1995 to about 80:1 in 2014.

Still, tracking THC potency over time can be tricky. The older a weed sample gets, the more its THC appears to degrade. How it is stored matters too. These two barriers could be interfering somewhat with the metrics on pot's potency.


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 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 to people who rarely or never used the drug, 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.

A small study of teenagers in Europe found that people who used marijuana had more gray matter in their brains, which can affect how humans mature over time.

The researchers noticed these brain-level changes in teens who had just one or two joints in their lifetimes.

Although researchers are unsure whether higher brain volume is bad for health, they do know brain volume naturally decreases during the aging process, according to the National Institutes of Health (NIH).

An international meta-analysis of 23,317 young adults also found that marijuana use could increase risk for depression, anxiety, and suicidal thoughts.

"Although the causes of major depressive disorder are multifactorial and complex, this meta-analysis suggests that the cannabis exposure could be 1 factor contributing to depression in young adulthood," the researchers wrote.


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 cigarettes.

Other forms of marijuana consumption, like vaping or eating edibles, have not been linked to increased cancer risk like smoking has been linked.

"When you combust any plant, you're creating significantly more carcinogens," Dr. Jeffrey Chen, the director of the UCLA Cannabis Research Initiative, previously told INSIDER.


Some athletes think marijuana could be used in ways that might improve certain types of physical performance.

Andy Cross/The Denver Post via Getty Images

Some athletes, especially in certain endurance and 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, since it affects coordination and motivation, and dulls the body's natural recovery process.

Without more research, it's hard to know 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 using marijuana during pregnancy could cause complications and increase the risk that a baby would have to spend time in a neonatal intensive care unit.

Still, pregnant and breastfeeding women have reported using the drug to relieve nausea, pain, and depression symptoms since traditional NSAID painkillers are not allowed during pregnancy.

Experts say they have little information about the potential health risks associated with this practice, so they urge mothers to abstain from marijuana consumption.


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 available evidence 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.

Staci Gruber, an associate professor of psychiatry at Harvard Medical School and the director of the Marijuana Investigations for Neuroscientific Discovery program at McLean Hospital, told Business Insider that marijuana research is essential in determining "how best we can use it, what are the safest ways, and what are the real risks."

Kevin Loria contributed to a previous version of this story.

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