A Big Study on Weed and Mental Health Reveals Just How Little We Know

The findings don’t mean that cannabis doesn’t help mental health—just that we need more research on the effects of weed on the mind.
person holding a nugget of weed and a woman lying on a bed
ZOA PHOTO/Stocksy, Mauro Grigollo/Stocksy

In April of 2019, more people searched online for CBD than for acupuncture, apple cider vinegar, meditation, exercise, veganism, or vaccination (and that was during a measles outbreak).

CBD, a chemical found in the cannabis plant, is having its moment, after being seized on by the wellness empire for its rumored ability to help with a whole host of conditions, from anxiety to insomnia to depression. The CBD industry is estimated to grow to almost $2 billion by 2022, and cannabis use overall has increased 43 percent between 2007 and 2015; it's now medicinally legal in 33 states. But despite how ubiquitous CBD lattes may be, they are not matched by an equal amount of research on their benefits for the mind.


A new review and meta-analysis published this week in The Lancet Psychiatry looked for the effects of cannabinoids on mental health in nearly 40 years of research and their findings sounded grim: They wrote there was “scarce evidence” to support that cannabis improves mental health symptoms, leading publications like The Guardian to publish an article titled "Risks of cannabis use for mental health treatment outweigh benefits," and write that "the use of cannabis medicines to treat people with depression, anxiety, psychosis or other mental health issues cannot be justified because there is little evidence that they work or are safe." Time similarly concluded that "There's 'Scarce Evidence' That Cannabis Helps Mental Health Issues."

This review reveals something many clinicians already knew: We don’t have enough evidence to say that cannabis can treat mental health disorders. That doesn’t necessarily mean weed doesn’t help at all—it means we just don't know. (And since risks of long-term cannabis use have been well-documented, of course they would outweigh benefits we are unaware of.)

“The old adage that absence of evidence doesn’t necessarily mean absence of effectiveness is true here,” said Harry Sumnall, a professor of substance use at Liverpool John Moores Public Health Institute in the U.K., who was not involved in the review.

Finding a lack of evidence isn't a reason to throw in the towel. It should be motivation to conduct more rigorous studies, especially given the rise in use of cannabis and cannabis-derived products specifically for mental health, and the large swaths of the public deciding on their own that cannabis does treat these symptoms.


People are widely using cannabis—both THC and CBD—for their mental health. In 2017, a study found that people perceive cannabis to be an effective way to treat many conditions, and that some substituted cannabis for prescription medications like benzodiazepines (often given for anxiety) or antidepressants. In a 2018 study of over 2,400 CBD users, 62 percent said they used CBD for a medical condition—the top three being pain, anxiety, and depression.

Just because your friend or your favorite Instagram influencer took CBD and it improved their anxiety better isn’t enough to determine whether cannabis is effective for that purpose. This kind of evidence is called anecdotal and it can feel immensely powerful, especially if the experience happened to you. But that’s not the way we decide that treatments are safe and effective. Even results from single studies might not be enough, especially when it comes to difficult areas like mental health. Consider the fact that researchers are still having debates about whether or not SSRIs are more effective than placebos for depression— and those medications have been around for decades and have no issues surrounding legality that result in limits on research. This is one of the reasons scientists write reviews and meta-analyses, to try and combine findings from many studies.

The authors of the new review searched for studies published between 1980 and 2018, including unpublished or ongoing studies, where medicinal cannabinoids were given to adults to treat depression, anxiety, attention-deficit hyperactivity disorder, Tourette's syndrome, post-traumatic stress disorder, or psychosis. In the end, they included 83 studies, 30 of which were randomized controlled trials—considered the gold standard of study design.


The results were mixed. "Our analyses and conclusions are limited by the small amount of available data, small study sizes, and heterogeneity of findings across studies," the authors wrote.

They found that pharmaceutical-grade THC made anxiety symptoms better, but only in people who had other medical conditions like chronic pain. This is an important caveat. If, for example, the primary outcome of a study was seeing if cannabis could help with chronic pain and a person’s depression also improved, it’s hard to say whether the cannabis treated the depression, or if their pain got better and made them feel less depressed.

In one study the review looked at, pharmaceutical cannabis made psychotic symptoms worse, while in others, pharmaceutical cannabis didn’t show any significant effect on mental disorders, but was linked to increased side effects. The authors noted that there were very few randomized controlled trials for them to review that tested pharmaceutical CBD or medicinal cannabis. Another issue is that many people don't take pharmaceutical or medicinal cannabis, they buy it recreationally—the studies can't account for that variability either.

“To make more confident conclusions we need more evidence; but at the moment there is not a lot that can support, guide or inform use of cannabinoids for mental disorders," said Louisa Degenhardt, the deputy director at the National Drug and Alcohol Research Centre at The University of New South Wales in Sydney, and senior author on the review.


Kevin Hill, an addiction psychiatrist at Beth Israel Deaconess Medical Center and an assistant professor of psychiatry at Harvard Medical School, said that any clinician who treats patients who regularly use cannabis, either recreationally or medicinally, won't be surprised at the mixed and sparse evidence the authors had to muddle through.

“There is much more that we don’t know about cannabis and CBD than we do know,” Hill said. “With such an intense interest in cannabis and CBD as treatments for medical conditions including psychiatric disorders, it is disappointing that the rate and scale [of research] has not kept pace with the interest.”

Cannabis is a Schedule 1 drug, which is a barrier to research, but funding is a bigger one, Hill said. He thinks that states and companies profiting from cannabis and CBD should contribute to the science. “For the most part, they have not," Hill said. "A portion of profits from the sale of cannabis or CBD should be put toward finding the answers to important questions about efficacy and safety." It's a process we know can work, even for something like CBD: Large-scale trials were how we found that CBD could be helpful for pediatric epilepsy conditions, and it's now FDA-approved for that use.

The reason why this all matters is because with mental health disorders, taking something that’s not helping could eventually end up doing harm. If someone with depression takes CBD or medical cannabis daily and it doesn't work (or makes it worse), they won't improve. This could affect their overall quality of life, and their ability to work or be social.


Without more study, we could also be missing some of the basic biology around cannabis use. Earlier this year, a small study looked at the medical records of 25 people who used cannabis and found that they needed more anesthesia to remain sedated during certain medical procedures. When the authors tried to look at existing research to see if other clinicians had found the same thing, they discovered that their study was the first on that topic. “We did these huge literature searches and found nothing,” Mark Twardowski, a doctor of osteopathic medicine, told VICE in April. “Really?”

And many CBD products continue to be notoriously under regulated: In 2017, a study in JAMA found that only 30 percent of CBD products sold online were accurately labeled, and last year, a study in Forensic Science International found synthetic marijuana and dextromethorphan, an ingredient in cough syrup, in CBD vape liquid.

"With millions of Americans using cannabis and CBD for myriad medical conditions, we should be conducting rigorously designed trials to see if cannabis and CBD actually are effective treatments for these conditions," Hill said. The United States has the potential to lead the way in this work and we have not yet done so.”

Until more research is done, we should be wary of overblown claims around cannabis on both sides: that it does nothing, or that it's a panacea—our gap in knowledge is too great for either.

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