ABOVE: © ISTOCK.COM, LPETTET

So far, research has failed to provide convincing evidence of the efficacy of medicinal cannabis treatments to alleviate symptoms associated with mental illnesses such as anxiety, depression, attention-deficit hyperactivity disorder, Tourette syndrome, post-traumatic stress disorder, or psychosis, according to a meta-analysis published in The Lancet Psychiatry yesterday (October 28).

The study, which examined 83 studies since 1980 involving a total of more than 3,000 people, also found concerns with the use of medical marijuana treatments that include the main psychoactive ingredient of cannabis, delta-9-tetrahydrocannabinol (THC). One 2005 study of 24 patients, for example, found that a treatment that included THC worsened the negative symptoms of psychosis and reduced cognitive functioning compared with placebo. And looking across nearly a dozen randomized controlled trials, other THC-based therapies led to more adverse side effects compared with placebo, and more patients withdrew from the study as a result.

“Cannabinoids are often advocated as a treatment for various mental health conditions,” coauthor Louisa Degenhardt, a drug and alcohol expert at Australia’s University of New South Wales, tells Reuters. “[But] clinicians and consumers need to be aware of the low quality and quantity of evidence . . . and the potential risk of adverse events . . . and until evidence from randomized controlled trials is available, clinical guidelines cannot be drawn up around their use in mental health disorders.”

See “State Marijuana Legalization Aids Research Very Little”

Deepak Cyril D’Souza of Yale University School of Medicine agrees. In a commentary published alongside the meta-analysis, he writes: “[I]n light of the paucity of evidence, the absence of good quality evidence for efficacy, and the known risk of cannabinoids, their use as treatments for psychiatric disorders cannot be justified at present.”

Jef Akst is the managing editor of The Scientist. Email her at jakst@the-scientist.com.