Trials of the controversial anti-malarial drug taken by US President Donald Trump to try to prevent catching coronavirus are to be resumed.
UK regulators say hydroxychloroquine and a similar drug chloroquine can be given to healthcare workers in a clinical study to test the theory.
That work looked at treating Covid-19.
It concluded the drug was not beneficial and increased the risk of irregular heart rhythms and death. That publication led to the WHO suspending its coronavirus treatment trials of the anti-malaria drug.
Concerns were raised about the data and then some of the study's authors said they could no longer stand by their publication in The Lancet as the healthcare firm Surgisphere that was involved in the work would not allow an independent review.
The New England Journal of Medicine retracted another paper that had data from Surgisphere.
The cheap and widely available drug has been safely used to prevent malaria infection for years.
Although studies suggest hydroxychloroquine is not a life-saver for people who are already ill with coronavirus, researchers are keen to continue exploring whether it might prevent infections.
The COPCOV trial will see chloroquine, hydroxychloroquine or a placebo given to more than 40,000 healthcare workers from Europe, Africa, Asia and South America.
One of the lead researchers, Prof Sir Nicholas White from the University of Oxford, said: "Hydroxychloroquine could still prevent infections, and this needs to be determined in a randomised controlled trial."
Co-investigator Prof Martin Llewelyn, from the Brighton and Sussex Medical School, said: "Although rates of coronavirus are low just now in the UK, healthcare workers are still being affected across the NHS and a second wave of infection this winter is widely expected.
"In terms of finding an intervention that could protect key workers by this winter, hydroxychloroquine is by far the most realistic prospect. The recent post-exposure prophylaxis study confirmed its safety and indicated that it could be protective if given as pre-exposure prophylaxis. This is what COPCOV will find out."
Meanwhile, the anti-HIV drugs lopinavir and ritonavir have been found not to provide any clinical benefit to patients hospitalised with coronavirus.
But the same RECOVERY trial recently discovered that a cheap steroid called dexamethasone can help save the lives of patients who are seriously ill with the virus.