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.
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.