Medical marijuana could finally be inching toward federal approval. After decades of advocacy from cannabis proponents and the marijuana industry, the Trump administration is moving to ease some cannabis restrictions and boost research on the drug’s therapeutic uses. Most U.S. states—40 in all—and the Washington, D.C., already allow medical marijuana as a treatment for dozens of conditions, from arthritis to inflammatory bowel disease, hepatitis C, cancer, glaucoma, Alzheimer’s, and more. But despite cannabis’s popularity, experts say that the scientific evidence as to whether it can actually treat many of these health issues is often thin—though promising.

“Some people will have you believe that it can help every condition,” says Jack Wilson, a postdoctoral research fellow at the Matilda Center for Research in Mental Health and Substance Use at the University of Sydney in Australia. “They think that it’s some sort of silver bullet, but that’s just not the case.”

Cannabis is inherently difficult to study. The plant, Cannabis sativa, contains hundreds of compounds, including more than 100 cannabinoids—and each of these may have their own potential health effects. Furthermore, people take cannabis in myriad different forms—flowers, waxes, edibles, tinctures, creams, suppositories, and more—and at varying doses. It has also been highly controlled on the federal level, where cannabis has been broadly considered to be in the same class of drugs as heroin and LSD. And that has made research hard to do and expensive because it has required labs to get extra federal permissions and to invest in extra layers of security.

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There is also a dearth of clinical evidence. Most of the available medical marijuana products on the market have not been tested in large-scale clinical trials—the gold standard for determining whether a treatment or intervention is safe and effective. “That’s why there’s so much confusion,” says Ryan Vandrey, a professor of psychiatry and behavioral sciences at Johns Hopkins University. “That’s why there’s such a lack of good clinical evidence.”

That’s not stopping people from using marijuana as medicine, however. “Societally, people are moving on with this as a medicine with very little data,” says Margaret Haney, director of the Cannabis Research Laboratory at Columbia University. “People say, ‘Talk to your doctor.’ Well, your doctors don’t know anything because they don't have the data.”

Scientific American spoke with several cannabis researchers to learn what potential medical marijuana treatments are backed by science, what areas of research are most promising and how a possible shift in federal regulations could help close some of the evidence gaps.

What does cannabis do to our body?

Smoking, vaping, eating or otherwise consuming cannabis can have various effects on the brain and body. Some of the most common include relaxation, as well as adverse reactions such as heightened anxiety or paranoia.

The psychoactive effect of cannabis largely arises from the cannabinoid tetrahydrocannabinol (THC), which binds to the so-called endocannabinoid system, a network of neural circuits throughout the body that help regulate things such as sleep, mood and brain function. Cannabinol (CBD), another commonly consumed cannabinoid, interacts with the endocannabinoid system, too, but in a less noticeable way.

How much of a cannabis product a person ingests, what their age and individual physiology are, what form of cannabinoids are in the drug and how they take it can all influence its effects. For instance, if a person inhales a cannabis product, they might feel the physiological effects within a few minutes, says Igor Grant, a distinguished psychiatry professor at the University of California, San Diego. But consuming cannabis in an edible—thus sending it through the gastrointestinal tract—may mean the effects will take longer to feel.

What uses of medical marijuana are best supported by evidence?

Cannabis has been used as a medical therapeutic since ancient times, with records of its use dating back to at least 2800 B.C.E. in China. Today in the U.S., however, very few medical cannabis products have been approved for use by the U.S. Food and Drug Administration. These include a CBD-based drug called Epidiolex, which can treat rare forms of childhood epilepsy, as well as three synthetic cannabinoid drugs, which can treat nausea in cancer patients and weight loss associated with AIDS.

In 2017 a report from the National Academies of Sciences, Engineering, and Medicine concluded that the strongest evidence for cannabis’s medicinal uses by that time included the treatment of chronic pain, chemotherapy-induced nausea and symptoms related to multiple sclerosis.

Some findings are promising—but more research is needed

For other potential uses of medical marijuana, the evidence is fuzzier.

A recent review of randomized controlled trials by Wilson and his colleagues found no evidence that cannabis can treat mental health conditions such as anxiety, anorexia nervosa or post-traumatic stress disorder. Curiously, the researchers did find some evidence that medical marijuana may be able to treat cannabis use disorder. (The premise is similar to how nicotine patches can help treat cigarette addiction.)

Wilson stresses, however, that the lack of evidence doesn’t mean research into whether medical marijuana can help treat these or other mental health conditions should stop: “The door isn't closed on a lot of these conditions,” he says. “We should definitely pursue more evidence.”

For example, some of Grant’s colleagues at the University of California, Davis’s Center for Medicinal Cannabis Research are conducting a clinical trial that suggested that CBD may help ease some of the underlying anxiety involved with anorexia, he says.

While far from conclusive, there is “increasing evidence” that CBD may help treat anxiety in some, Grant says. “If it turns out to be true, it’s important,” he adds. CBD has a “pretty good safety profile,” which could mean that people who use it to treat anxiety may be able to rely less on drugs such as antidepressants and benzodiazepines, which can carry negative side effects or be habit-forming. But more research is needed.

There is also some preliminary evidence that suggests that CBD could be used to treat schizophrenia with fewer side effects than antipsychotic drugs, which can cause weight gain and neurological symptoms, for example, Grant says.

Other potential directions for research include studying the possible anti-inflammatory effects of cannabinoids, as well as THC and CBD’s role in managing metabolic syndrome, a precursor to heart disease, type 2 diabetes and stroke, he says.

How experts say a change in federal laws could help

In April the Trump administration officially rescheduled state-licensed medical marijuana under the Controlled Substances Act (CSA) from Schedule I—a category of drugs, such as heroin, that the government views as the most dangerous and least medically useful—to Schedule III, a less dangerous category. The administration also plans to consider broadly reclassifying all cannabis to Schedule III. That wouldn’t legalize marijuana, but researchers say it would lighten the burden of studying it.

“The movement from Schedule I to Schedule III is really important because it opens a lot of research doors,” Johns Hopkins’ Vandrey says. “It’s not that none of this research could be done before, but it moving to Schedule III makes it easier to do the research.”

That’s because, in the eyes of the federal government, cannabis had to be treated as carefully as heroin. Researching cannabis involves navigating obstacles such as paperwork and following strict rules to ensure that it is stored securely and used as designed.

“To study cannabis as a Schedule I drug, I have a gun safe in a locked room that the [Drug Enforcement Agency] approves and that only I can get in with my fingerprints,” says Columbia’s Haney. Reclassifying cannabis would have a “pretty profound impact” on the number of people who can study it, she says—and by extension, that could expedite sorely needed clinical research.

“Right now we have the notion of medical cannabis really being driven by the industry,” Haney says. “They’re running the narrative here because science cannot keep up with what needs to be done.”