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The benefits and harms of marijuana, explained by the most thorough research review yet

A new report looks at more than 10,000 studies on marijuana. It has good and bad news for pot users.

Marijuana has been with humans in some way or another for thousands of years. But after all this time, there is still a lot of public debate about what, exactly, pot’s risks and benefits are.

A recent review of the research from the National Academies of Sciences, Engineering, and Medicine attempts to fill the gap in our knowledge. By combing through more than 10,000 studies published since 1999, the review, conducted by more than a dozen experts, provides the clearest look at the scientific evidence on marijuana yet.

The research finds both some strong benefits and major downsides to cannabis. It seems to be promising for chronic pain, multiple sclerosis, and cancer patients. But it also seems to pose a significant risk for respiratory problems if smoked, schizophrenia and psychosis, car crashes, lagging social achievement in life, and perhaps pregnancy-related problems.

The findings aren’t just for marijuana; they’re for marijuana or cannabinoids, chemical compounds commonly found in pot. It’s possible that, down the line, some of the benefits in particular will be split from the marijuana leaf itself — although many drug experts believe that there’s an “entourage effect” with marijuana in which all of its cannabinoids and chemicals, which number in the hundreds, work together to make its effects as potent as possible.

One major caveat to this: The report is, by its own admission, only a best guess for a lot of its findings, because much of the research out there just isn’t very good. The report pins the lack of good research largely on government policies — particularly regulatory barriers linked to marijuana’s federal classification as a highly restricted Schedule 1 substance — that make it hard to conduct good studies on the drug. The National Academies ultimately calls for these barriers to be cut down and more research to be funded so we can get a better idea of what pot is capable of, especially as more states legalize it for both medical and recreational uses.

Still, the report is the best look at marijuana yet. It is nearly 400 pages; if you want a really deep dive into the benefits and harms of marijuana, you should read it in full. But here I’ve provided a summary of what the researchers found.

What are marijuana’s benefits?

A marijuana plant. Shutterstock

Since the mid-1990s, 28 states have legalized marijuana for medical uses. But in all that time, the benefits of pot have remained hazy. Despite some research showing that it can be good for pain and muscle stiffness, many of the claims about what pot can do for other ailments — such as epilepsy and irritable bowel syndrome — are based on anecdotal evidence and have yet to be scientifically proven.

The report can’t fully validate or invalidate all of the claims about marijuana’s medical benefits, given that there are still no studies on some of these questions, and many of the studies that are out there are bad or lacking. But it does have some solid findings.

For one, the review confirms what previous studies have found: There is “substantial evidence” that marijuana is good for treating chronic pain. This is one of the most common reasons cited for marijuana’s medical use — particularly in light of the opioid painkiller epidemic, which has spawned in part as patients turn to opioids to try to treat debilitating pain. The report concludes that marijuana can treat chronic pain. And that may allow it to substitute more dangerous, deadlier opioid painkillers.

The report also found “conclusive evidence” that marijuana is effective for treating chemotherapy-induced nausea and vomiting. Coupled with the findings on pain, this suggests that marijuana really is a potent treatment for cancer patients in particular, who can suffer from debilitating pain and severe nausea as a result of their illness.

And the report found “substantial evidence” that marijuana can improve patient-reported multiple sclerosis spasticity symptoms. But it only found “limited evidence” for marijuana improving doctor-reported symptoms of this kind.

Beyond the strongest findings, the report found “moderate evidence” that marijuana is effective for “improving short-term sleep outcomes in individuals with sleep disturbance associated with obstructive sleep apnea syndrome, fibromyalgia, chronic pain, and multiple sclerosis.” It also found “limited evidence” for marijuana’s ability to treat appetite and weight loss associated with HIV/AIDS, improving Tourette syndrome symptoms, improving anxiety symptoms in individuals with social anxiety disorders, and improving PTSD. And there’s “limited evidence” of a correlation between marijuana and better outcomes after a traumatic brain injury.

The report also disproved — or at least cast a lot of doubt — on some of the claimed benefits of pot. It found “limited evidence” that marijuana is ineffective for treating symptoms associated with dementia and glaucoma, as well as depressive symptoms in individuals with chronic pain or multiple sclerosis.

And it found “no or insufficient evidence” for marijuana as a treatment for cancers, cancer-associated anorexia, irritable bowel syndrome, epilepsy, spasticity in patients with paralysis due to spinal cord injury, amyotrophic lateral sclerosis, Huntington’s disease, Parkinson’s disease, dystonia, drug addiction, and schizophrenia. This doesn’t mean that marijuana can’t treat any of these — some patients, who are prescribed pot for these ailments today, will swear that marijuana helped treat their epilepsy, for example — but that there’s just not enough evidence so far to evaluate the claims.

Overall, the report suggests that, as far as therapeutic benefits go, marijuana is a solid treatment for multiple symptoms associated to chronic pain, chemotherapy-induced nausea and vomiting, and multiple sclerosis. Everything else, from epilepsy to HIV/AIDS, needs more research before pot is more definitively shown to be effective or ineffective.

What are marijuana’s harms?

Purple marijuana plants. Seth McConnell/Denver Post via Getty Images

Marijuana is often described as one of the safest drugs out there, in part because it’s never been definitively linked to an overdose death and it’s broadly safer than other drugs like alcohol, tobacco, cocaine, and heroin. And while the National Academies’ report doesn’t find evidence of a marijuana overdose death, it does add a few wrinkles to the narrative of marijuana as a safe drug.

For one, the report finds “substantial evidence” of marijuana’s negative effects for a few conditions. For long-term marijuana smokers, there’s a risk of worse respiratory symptoms and more frequent chronic bronchitis episodes. For pregnant women who smoke pot, there’s a risk of lower birth weight for the baby. For marijuana users in general, there’s a greater risk of developing schizophrenia and other psychoses. And there’s a link between marijuana use and increased risk of car crashes.

The report also found “limited evidence” of links between marijuana use and several other negative outcomes, including an increased risk of testicular cancer, triggering a heart attack, chronic obstructive pulmonary disease, and pregnancy complications. And it found “moderate” to “limited” evidence that marijuana use might worsen symptoms or risk for some mental health issues, including depressive disorders, bipolar disorder, suicidal ideation and suicide attempts among heavier users, and anxiety disorders, particularly social anxiety disorder among regular users.

Besides medical conditions, the report found evidence for some psychosocial problems. There’s “moderate evidence” that acute marijuana use impairs learning, memory, and attention. There’s “limited evidence” of marijuana use and worse outcomes in education, employment, income, and social functioning.

There was some good news: The report found “moderate evidence” of no link between marijuana smoking and lung cancer or marijuana use and head and neck cancers, which are commonly linked to tobacco. There was also “moderate evidence” of better cognitive performance among individuals with psychotic disorders and a history of marijuana use.

The report, however, couldn’t find sufficient evidence for pot’s links to a lot of problems: other types of cancer, an increased chronic risk of heart attack, asthma, later outcomes for infants born of mothers that used marijuana during pregnancy, deadly pot overdoses, and PTSD.

With the problems specifically linked to smoking marijuana, it’s worth noting that other forms of consumption — vaping and edibles in particular — may not carry the same risk. More research will be needed to evaluate that, particularly for vaping.

The report also found some “substantial evidence” that more pot use can lead to problematic marijuana use — what one typically thinks of as excessive use or even dependence. It also outlined, with “limited” to “substantial” evidence, some of the risk factors for problematic marijuana use, including being male, smoking cigarettes, a major depressive order, exposure to combined use of other drugs, and use at an earlier age. But it also cited “limited” to “moderate” evidence to rule out a few risk factors, including anxiety, personality, and bipolar disorders, adolescent ADHD, and alcohol or nicotine dependence.

It also found a “limited” to “moderate” evidence of a correlation between marijuana use and use of other illicit drugs. This is the typical evidence cited for the so-called “gateway” effect: that marijuana use may lead to the use of harder drugs.

One caveat to much of the research: correlation is not always causation. For example, in the case of the “gateway” effect, other researchers argue that the correlation between pot and harder drug use may just indicate that people prone to all sorts of drug use only start with marijuana because it’s the cheapest and most accessible of the illicit drugs. If cocaine or heroin were cheaper and more accessible, there’s a good chance people would start with those drugs first.

Still, the bottom line is that marijuana does pose some harms — particularly for people at risk of developing mental health disorders, pregnant women, those vulnerable to respiratory problems, and anyone getting into a car. And while some of these harms may be overcome by marijuana’s benefits or curtailed by consuming pot without smoking it, the evidence shows that weed’s reputation as a safe drug is undeserved.

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