Cannabis is seen as a potential mental health treatment Konstantinos Tsakalidis/Bloomberg/Getty

A major study has found little evidence that cannabis helps with depression, anxiety and other mental health conditions, despite growing claims that the drug is a useful treatment.

There is an urgent need for new and better mental health treatments. Cannabis has emerged as a potential option now that several countries have relaxed their laws against it.

Compounds like tetrahydrocannabinol (THC) and cannabidiol (CBD), which are found in cannabis and are two of a group of chemicals known as cannabinoids, are sometimes touted as a cure-all. Yet the scientific basis for their mental health benefits has been unclear.


To find out more, Wayne Hall at the University of Queensland, Australia, and his colleagues evaluated all the published and unpublished research between 1980 and 2018 on the use of cannabinoids to treat depression, anxiety, post-traumatic stress disorder and psychosis. They also included studies on the use of cannabinoids to treat symptoms of attention deficit hyperactivity disorder and Tourette syndrome.

Of the 83 studies, which included a total of 3000 participants, only 40 were randomised controlled trials, the gold standard for medical evidence.

Hall and his colleagues found “very low-quality” evidence that pharmaceutical THC led to small improvements in anxiety symptoms among patients with other conditions, primarily chronic non-cancer pain and multiple sclerosis. This could be because the drug helped with symptoms of those medical conditions, says Hall.

They found little evidence that medicinal cannabinoids helped to treat either the overall disorders or their individual symptoms. In one study of 24 people, THC actually made symptoms of psychosis worse.

“The popular media has been remarkably uncritical of the claims made for medical cannabis by the cannabis companies producing and marketing it,” says Hall. But the findings suggest that without high-quality evidence, treating mental disorders with cannabis isn’t justifiable.

Claims by medicinal cannabis companies should be treated with the same scepticism as those about new pharmaceutical drugs are, he says.

Despite decriminalisation in many countries , it can still be challenging for researchers to conduct high-quality studies into the plant’s possible therapeutic effects, says David Castle at the University of Melbourne, Australia.

Larger studies are needed to tease apart how different compounds and doses affect different conditions, he says. “The cannabis plant is a very complicated one that has many, many chemicals in it.”

Journal reference: The Lancet Psychiatry, DOI: 10.1016/S2215-0366(19)30401-8