In a review of scientific studies that analyzed the impact of medicinal cannabinoids on six mental health disorders, the researchers found “a lack of evidence for their effectiveness.”

Their findings have important implications for countries such as the United States, Australia, Britain and Canada, where medical cannabis is being made available for patients with certain illness, said Louisa Degenhardt, a drug and alcohol expert at Australia’s University of New South Wales in Sydney.

“There is a notable absence of high-quality evidence to properly assess the effectiveness and safety of medicinal cannabinoids … and until evidence from randomized controlled trials is available, clinical guidelines cannot be drawn up around their use in mental health disorders,” she said as her results were published in The Lancet Psychiatry journal.

Despite a lack of clinical trial evidence, anecdotally some military veterans and others who suffer post-traumatic stress disorder (PTSD), depression and anxiety say they have found cannabis helpful in easing some of their symptoms. Other conditions cannabis is used for include nausea, epilepsy, and traumatic brain injury, but this study did not examine its impact on those.

Medicinal cannabinoids include medicinal cannabis and pharmaceutical cannabinoids, as well as their synthetic derivatives, THC, or delta-9-tetrahydrocannabinol – the main psychoactive ingredient of cannabis – and cannabidiol, or CBD.

“Cannabinoids are often advocated as a treatment for various mental health conditions,” Degenhardt said. “(But) clinicians and consumers need to be aware of the low quality and quantity of evidence … and the potential risk of adverse events.”

Degenhardt’s team sought to look at all available evidence for all types of medicinal cannabinoids. They included all study designs and investigated the impact on remission from and symptoms of depression, anxiety, attention-deficit hyperactivity disorder (ADHD), Tourette syndrome, PTSD and psychosis. They analyzed 83 published and unpublished studies covering around 3,000 people between 1980 and 2018.

They found that pharmaceutical THC – either with or without CBD – made psychosis worse, and did not significantly affect any other primary outcomes for the mental illnesses analyzed.

It also increased the number of people who reported side effects, and the number who decided to withdraw from a study due to side effects.

Tom Freeman, an addiction and mental health expert at Britain’s Bath University who was not involved with the study, said the findings highlighted an urgent need for high-quality trials of medical cannabis to strengthen the evidence – particularly given what he said was “significant demand” from patients.