A large umbrella review has found no strong evidence to date that medicinal cannabis can effectively treat several mental health disorders, including anxiety, psychotic disorders, or post-traumatic stress disorder (PTSD).
An absence of evidence is not evidence of absence, but the findings do underline just how much more research needs to be done to understand medicinal cannabis and ensure prescribed drugs are both safe and effective for the individual.
The review, led by researchers at the University of Sydney, Australia, included health data from 2,477 participants across 54 randomized controlled trials on medicinal cannabis, conducted between 1980 and 2025.
Overall, it reveals "relatively weak evidence" that cannabinoids were beneficial compared to a placebo for most mental health conditions examined. Still, that may be a reflection of weak methodologies used in the past, more than cannabis' actual performance.
The review is the largest analysis of its kind to date, according to the authors, and yet it included just eight trials on psychotic disorders, six trials on anxiety disorders, and three on PTSD.
There was insufficient data to meta-analyze studies exploring cannabinoid treatments for ADHD, bipolar disorder, OCD, and tobacco use disorder. What's more, there was no eligible randomized controlled trial testing how useful cannabinoids are for treating depression.
A little over half of the trials included in the review only followed up with participants for less than a month, and cannabis products were typically taken orally, via droplets or tablets.
Some of the drugs being tested contained THC, while others contained compounds like CBD or a mix, making them difficult to compare. It's also challenging to double-blind these trials, as participants taking THC rather than a placebo often know because of the psychoactive effects.
Another hurdle lies within broad categories like 'anxiety'. Some of the trials included in this review focused on patients with generalized anxiety, while others focused on those with social anxiety.
If cannabis does have a positive impact on mental health, the drug is unlikely to work the same way for everyone, which means that future research needs to be very specific.
If nothing else, this review shows that "there is a crucial need for randomized controlled trials with larger and more representative samples."
Depression, anxiety, and PTSD are three of the most common mental health conditions in the world, and they encompass a huge diversity of complex, interacting symptoms of varying severity. The way patients respond to standard treatments is highly varied and often requires years of trial and error.
Antidepressant drugs and cognitive behavioral therapy are first-line options to treat anxiety and PTSD, but their effects often take weeks or longer to kick in, and while they do help many people, a substantial proportion of patients experience only partial benefit or no benefit at all. Side effects and tolerability can also limit their use.
With few other options, a growing number of patients are turning to cannabis-based treatments.
Some scientists are concerned by this, as the impact of cannabis on mental health remains poorly understood, and in some cases, it's possible the drug may exacerbate symptoms of anxiety or psychosis.
Other patients with PTSD, anxiety, or other mental health disorders, however, report finding medicinal cannabis helpful in alleviating their symptoms.
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Disagreement reigns on how to balance the pros and cons of cannabis for conditions that do not respond to available treatments, and legislation varies from nation to nation.
When it comes to historically illicit substances like cannabis and other psychoactive drugs, such as ketamine, psilocybin, and ecstasy, the potential benefits to health are only now being put to the test, as are the potential risks.
Far more research is needed to understand what impacts these drugs have on the brain and body, and how they perform against placebos and standard treatments.
The study was published in The Lancet Psychiatry.