The use of cannabis medicines to treat people with depression, anxiety, psychosis or other mental health issues cannot be justified because there is little evidence that they work or are safe, The Guardian reports citing a major new study.

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.

The authors looked for evidence of an effect of medicinal cannabinoids in trials conducted in depression, anxiety, attention-deficit hyperactivity disorder (ADHD), Tourette syndrome, post-traumatic stress disorder (PTSD) and psychosis. They found 83 studies in 3,000 people. But the evidence of a positive effect was sparse.

There was, they said, “low-quality evidence” that THC, one of the derivatives of cannabis, could be helpful to people with multiple sclerosis or chronic pain who were also suffering from anxiety. But it made people with psychosis worse.

While there is little evidence that cannabinoids can help, the authors found more evidence in “a large body of research” of the potential harms. “This research suggests that cannabis use can increase the occurrence of depression, anxiety, and psychotic symptom,” says the paper.

A well-constructed study found that smoked cannabis actually increased the risk of acute psychotic symptoms. And young adults, the age group more likely to suffer from anxiety, depression and psychosis, are more likely to become dependent if they use cannabis daily over a long period of time.