A review of evidence from trials conducted over nearly 40 years, published in the journal Lancet Psychiatry, concludes that the risks outweigh the benefits. And yet, say the authors, they are being given to people with mental health problems in Australia, the US and Canada, and demand is likely to grow.

Prof Louisa Degenhardt of the National Drug and Alcohol Research Centre at UNSW Sydney, Australia, lead author of the study, said the findings had important implications in countries where medical use was allowed.

“There is a notable absence of high-quality evidence to properly assess the effectiveness and safety of medicinal cannabinoids compared with placebo, and until evidence from randomised controlled trials is available, clinical guidelines cannot be drawn up around their use in mental health disorders,” she said.