Scientists conducted a small trial of people with psychosis and found patients treated with CBD had lower levels of psychotic symptoms than those who received a placebo. Psychosis is characterised by paranoia and hallucinations.

The study found that they were also more likely to be rated as "improved" by their psychiatrist and there were signs of better cognitive performance and functioning.

The most common forms of psychosis are part of mental illnesses such as schizophrenia - which affects more than 21 million people worldwide - and bipolar disorder, but psychotic symptoms can also occur in conditions like Parkinson’s disease and alcohol or drug abuse.

The main psychoactive ingredient in cannabis is delta-9-tetrahydrocannabinol, or THC. It can induce paranoia and anxiety and hallucinations and has been found in studies to increase the risk psychotic illness in people who regularly use potent forms of cannabis such as skunk.

But its second major constituent, CBD, has the opposite effects to THC - leading scientists to think it might one day be useful as a treatment in mental health.

Scientists at King's College London's Institute of Psychiatry, Psychology & Neuroscience conducted a placebo-controlled trial of CBD in patients with psychosis and published their findings in the American Journal of Psychiatry.

In the trial, 88 patients with psychosis received either CBD or placebo for six weeks, alongside their existing antipsychotic medication. Beforehand and afterwards, the scientists assessed symptoms, functioning and cognitive performance, and the patients' psychiatrists rated their overall condition overall.

"The study indicated that CBD may be effective in psychosis: patients treated with CBD showed a significant reduction in symptoms, and their treating psychiatrists rated them as having improved overall," said Philip McGuire, who co-led the trial.

He noted that trial patients also reported few adverse side effects, and added: "Although it is still unclear exactly how CBD works, it acts in a different way to antipsychotic medication, and .. could represent a new class of treatment."