Dozens of studies involving more than 3,000 people did not provide compelling evidence that medical cannabis can help treat disorders of the mind, the review authors concluded.
"Cannabinoids are often advocated as a treatment for various mental disorders," said senior researcher Louisa Degenhardt, deputy director of the National Drug and Alcohol Research Center at the University of New South Wales in Sydney, Australia. "Clinicians and consumers need to be aware of the low quality and quantity of evidence for the effectiveness of medicinal cannabinoids in treating mental disorders and the potential risk of adverse events."
Medical pot might even make matters worse, the study authors said. Those adverse events can include worsening the occurrence of depression, anxiety and psychotic symptoms, the researchers found.
But advocates for marijuana law reform say the real lesson to be drawn from the evidence review is that medical pot remains under-researched.
"The absence of compelling evidence in favor of these cannabinoids appears to stem primarily from the absence of good research. Absence of evidence is not evidence of absence," said Mitch Earleywine. He's a professor of psychology at the University at Albany, State University of New York, and an advisory board member of NORML (the National Organization for the Reform of Marijuana Laws), a nonprofit group.
The legalization of medical marijuana has spurred interest from patients that's been growing in recent years, said Yasmin Hurd, director of the Addiction Institute of Mount Sinai, in New York City.
"At the end of the day, people are suffering and they're not getting a benefit from their current medication, or they're afraid of their current medication," Hurd said. "Everyone wants to try something new, and everyone's talking about medical cannabis, and people are just searching for an effective treatment."
A 2017 report from the National Academy of Sciences found there's enough evidence to say that marijuana products are effective at treating chronic pain, calming muscle spasms caused by multiple sclerosis, and easing nausea from chemotherapy.
Degenhardt and her colleagues decided to focus instead on the potential for cannabis to treat mental disorders and symptoms.
The researchers combed the medical literature and found 83 studies, including 40 clinical trials, to include in their review. These studies and trials included 42 for depression, 31 for anxiety, 12 for post-traumatic stress disorder (PTSD), 11 for psychosis, eight for Tourette's syndrome and three for attention-deficit/hyperactivity disorder (ADHD).
Most studies involved the use of pharmaceutical THC, the chemical in pot that causes intoxication, the researchers noted. The THC may or may not have been accompanied by CBD, another marijuana chemical that's been the focus of recent medical research.
The most positive evidence came from people with other medical conditions who found that their anxiety improved with the use of marijuana or cannabis-derived products laced with THC, according to the report.
But medical marijuana did not significantly affect any other primary outcomes for the mental disorders examined, the researchers concluded.
Hurd said, "This study emphasized what we all know, that there isn't substantial proof that cannabis itself can treat all of these mental disorders."
It's possible that cannabis alleviated anxiety because the condition for which people were being treated had some improvement, the authors noted. For example, a person's chronic pain might have eased, helping them feel more relaxed.
On the other hand, medical pot increased the number of people who had adverse events in 10 studies, and in 11 studies it increased the number of people who withdrew from the study due to negative side effects.
Most importantly, the gaps in knowledge regarding the benefits of medical cannabis are glaring, Degenhardt concluded.
"Given the large amount of interest that many people express in the use of cannabinoids for mental health, our greatest surprise - and cause for concern, given its increasing use - was about the lack of data that provide guidance about which mental health concerns, and for which groups of people, cannabinoids might in fact be useful," Degenhardt said. "This gap needs to be filled."
"I hope that this paper will inspire better work, including large-sample randomized clinical trials where cannabinoids serve as a treatment for people whose primary diagnoses include each of the disorders mentioned," Earleywine said. "Unfortunately, I would guess that headlines worldwide will claim directly or indirectly that cannabinoids are no help, but in fact, we don't know if they do or not."
The new review was published online Oct. 28 in The Lancet journal.