Psychosis is characterised by paranoia and hallucinations. An ingredient in cannabis called cannabidiol or CBD has shown promise in a clinical trial as a potential new treatment for psychosis.
The main treatment for psychosis is antipsychotic drugs, which have been the first line of treatment since the 1950s. These drugs are usually partially effective, but in around a third of patients they don't work at all. They also can have significant side effects.
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.
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.
"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."