Marijuana’s official designation as a Schedule 1 drug in the US — something with “no currently accepted medical use” — means it’s pretty tough to study.
Yet 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.
Along with several other recent studies, a massive report released this year by the National Academies of Sciences, Engineering, and Medicine helps sum up exactly what we know — and what we don’t — about the science of weed.
One of weed's active ingredients, tetrahydrocannabinol, or THC, interacts with the brain's reward system, the part 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.
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.
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.
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.
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.
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.
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.
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.
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 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.
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.
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.
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.
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.
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.
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.
Some athletes think marijuana could be used in ways that might improve certain types of physical performance.
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.
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.
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 minimise 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.'
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