Evidence is weak for whether medicinal cannabis treatments can relieve mental illnesses such as anxiety, depression and psychosis, and doctors should prescribe them with great caution, researchers said on Monday.

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.