A trial to see whether two anti-malarial drugs could prevent Covid-19 has begun in Brighton and Oxford.
Chloroquine, hydroxychloroquine or a placebo will be given to more than 40,000 healthcare workers from Europe, Africa, Asia and South America.
All the participants are staff who are in contact with Covid-19 patients.
US President Donald Trump was criticised this week after he said he had been taking hydroxychloroquine, despite warnings it might be unsafe.
The first UK participants in the global trial are being enrolled on Thursday at the Brighton and Sussex University Hospitals and the John Radcliffe Hospital in Oxford.
They will be given either hydroxychloroquine or a placebo for three months. At sites in Asia, participants will be given chloroquine or a placebo.
These are the first of a planned 25 UK sites, with results expected by the end of the year.
The trial is open to anyone delivering direct care to coronavirus patients in the UK, as long as they have not been diagnosed with Covid-19.
It will test whether the drugs can prevent healthcare workers exposed to the virus from contracting it.
'Beneficial or harmful'
One of the study's leaders, Prof Nicholas White at the University of Oxford said: "We really do not know if chloroquine or hydroxychloroquine are beneficial or harmful against Covid-19."
But, he said, a randomised controlled trial such as this one, where neither the participant nor the researchers know who has been given the drug or a placebo, was the best way to find out.
"A widely available, safe and effective vaccine may be a long way off," said Prof Martin Llewelyn from Brighton and Sussex Medical School, who is also leading the study.
"If drugs as well-tolerated as chloroquine and hydroxychloroquine could reduce the chances of catching Covid-19, this would be incredibly valuable."
The drugs can reduce fever and inflammation and are used as both a prevention and a treatment for malaria.
Hydroxychloroquine regulates the body's immune response and is also used in the treatment of rheumatoid arthritis and Lupus - an inflammatory disease caused by an overactive immune system.
Lupus charities in the UK and US have raised concerns that demand for the drug associated with coronavirus could threaten the supply for patients who already rely on it.
The drug gained attention after US President Donald Trump suggested it may be beneficial, and this week said he was taking hydroxychloroquine to ward off coronavirus.
The US Food and Drug Administration warned against use of the medication outside hospitals, where the agency has granted temporary authorisation for its use in some cases, or clinical trials.
While the University of Oxford trial is taking place in a controlled clinical environment, the World Health Organization has warned that some individuals were self-medicating and risked causing themselves serious harm.
It has not yet been shown to be safe and effective in the prevention or treatment of coronavirus and can cause dangerous heart arrhythmias.
The trial also involves researchers from the UK, Thailand, Vietnam, Laos, Cambodia and Italy.