Oxford Uni is leading tests of the drug touted by Donald Trump, to see if it prevents coronavirus.
Health workers in Oxford, who work with coronavirus patients, are being enrolled into a new clinical trial to see if hydroxychloroquine or chloroquine prevents Covid-19.
Testing is open to staff at the John Radcliffe in Oxford and a hospital in Brighton,.
The investigation is being led by the Bangkok-based Mahidol Oxford Tropical Medicine Research Unit (MORU), which is supported by the University of Oxford and charity Wellcome.
The randomised, placebo-controlled trial will enrol more than 40,000 people who work with confirmed or suspected coronavirus patients from Europe, Africa, Asia and South America.
MORU co-principal investigator Professor Sir Nicholas White said: "We really do not know if chloroquine or hydroxychloroquine are beneficial or harmful against Covid-19," he said.
"The best way to find out if they are effective in preventing Covid-19 is in a randomised clinical trial."
It comes just days after the US president's decision to take hydroxychloroquine was described as "a staggering, irresponsible act that could very well also amount to self-harm" and there are fears his actions risk running down supplies of the drug for people with other conditions who need it.
An Oxford Uni Professor warned earlier this week that there was no proof the anti-malaria drug works in treating Covid-19.
Dani Prieto-Alhambra also told us that the side-effects can be dangerous, and there were 'known issues' with people taking hydroxychloroquine for a long time, or combining it with other treatments.
But those running the MORU trial have said chloroquine and hydroxychloroquine "could reduce the chances" of catching coronavirus amid fears of a second wave of infections.
The price of Hydroxychloroquine is said to have risen dramatically as the availability of the drug has reduced because of demand from those who believe it will prevent Covid-19.
The drug is a prescription drug used for acute malaria and certain types of arthritis, which can reduce inflammation, pain, and swelling - and is widely used to treat rheumatic diseases.
It is a derivative of chloroquine, which is also used to treat malaria.
The UK Government has said that chloroquine and hydroxychloroquine are not licensed to treat Covid-19 related symptoms or prevent infection.
It said the drugs should not be used outside ongoing clinical trials which have reached no conclusions over the safety and effectiveness of the medicine on coronavirus.
After recruiting in Oxford and Brighton, further testing is expected at another four sites by the end of May, with 25 locations opened across the UK before July and more planned around the globe.
The team aims to deliver results by the end of 2020.