A major study is being launched to see if people with psychosis or psychotic symptoms could benefit from cannabidiol (CBD).
CBD is the second most abundant active ingredient in marijuana, but it does not provide a high on its own, and there is no evidence that people become addicted.
The NHS only uses it for a few conditions, such as severe epilepsy, people with certain symptoms of multiple sclerosis, and people with vomiting and nausea caused by chemotherapy.
However, it is also thought to help reduce anxiety and insomnia.
A £16.5m grant from the Wellcome Trust to the Department of Psychiatry at the University of Oxford will now be used to launch a global study involving 1,000 people.
Some of the people involved will be at high risk for psychosis, others will be patients who have already had their first episode of psychosis or those who have not responded to other treatments.
Thirty-five centers will be involved, mainly in Europe and North America, and the CBD will be provided free of charge by a pharmaceutical company.
Professor Philip McGuire, who will lead the research, said CBD was “one of the most promising new treatments” for people with psychosis.
“Many psychiatric patients are open to trying cannabidiol, and previous small studies have shown it has beneficial effects,” he said.
“In addition to treating established psychosis, this study will investigate whether cannabidiol can prevent psychotic episodes in high-risk individuals.
“This research could give us a new treatment for psychiatric disorders, and we are grateful to Wellcome and Jazz Pharmaceuticals for helping to make this happen.”
The cannabidiol used in the study was Epidyolex, a liquid approved for use in certain children and adults with epilepsy.
Lynsey Bilsland, from the Wellcome Trust, said it was important to try new treatments because traditional antipsychotics sometimes had “significant side effects” and didn’t work for everyone.
Researchers will also try to identify “biomarkers — biological landmarks — that would indicate that patients are likely to respond well,” she said.
Health shops in the UK can also legally sell CBD products such as oils, capsules and skin care products.
However, the NHS says they often contain very small amounts, so there is no guarantee of their quality or potential benefits.