Just hours after the WHO announced that clinical trials of hydroxychloroquine will resume, the first major study into the drug's effectiveness reported that it was no better than a placebo in preventing symptoms of Covid-19 developing.
Hydroxychloroquine has been controversially endorsed by public figures including US President Donald Trump despite a lack of evidence that it is effective.
University of Minnesota researchers tested 821 people who had recently been exposed to the virus or lived in a high-risk household.
It found 11.8% of subjects given hydroxychloroquine developed symptoms compatible with Covid-19, compared with 14.3% who got a placebo. That difference was not statistically significant, meaning the drug was no better than placebo.
"Our data is pretty clear that for post exposure, this does not really work," said Dr David Boulware, the trial's lead researcher and an infectious disease physician at the University of Minnesota.
The World Health Organization had announced earlier today that clinical trials of the drug would resume, having been suspended pending a safety review in the search for coronavirus treatments.
That decision followed The Lancet issuing an "expression of concern" over the large-scale study of hydroxychloroquine and chloroquine it published that led to the WHO suspending clinical trials of the anti-viral drugs as a potential treatment.
The study's publication last month triggered the WHO to announce it was pausing clinical trials of the drugs. France was among the countries to also halt Covid-19 treatment with hydroxychloroquine.
Several trials of the drug had been stopped over concerns about its safety for treating patients but the new trial found no serious side effects or heart problems from use of hydroxychloroquine.
"I think both sides - one side who is saying 'this is a dangerous drug' and the other side that says 'this works' -neither is correct," said Dr Boulware.