Anti-viral drug remdesivir has little to no effect on Covid patients' chances of survival, a study from the World Health Organization (WHO) has found.
The WHO trial evaluated four potential medications for Covid-19, including remdesivir and hydroxychloroquine.
Remdesivir was among the first to be used to treat coronavirus, and was recently given to US President Donald Trump when he was in hospital.
The drug's manufacturer Gilead rejected the findings of the trial.
In a statement, Gilead said the findings of the study were "inconsistent" with others, and that it was "concerned" that the results have yet to be reviewed.
What did the WHO study find?
For its Solidarity clinical trial, the WHO tested the effects four potential treatments - remdesivir, an Ebola drug, was one, but they also looked at malaria drug hydroxychloroquine, auto-immune drug interferon, and the HIV drug combination of lopinavir and ritonavir.
Dexamethasone, a low-cost steroid now widely used on Covid patients in intensive care in the UK, was not included in this study.
The four drugs were tested with 11,266 adult patients in total, across 500 hospitals in more than 30 different countries.
The results, which are yet to be peer-reviewed, suggest that none of these treatments has a substantial effect on mortality or on the length of time spent in hospital, the WHO said on Thursday.
WHO chief scientist Soumya Swaminathan said on Wednesday that their trials on hydroxychloroquine and lopinavir/ritonavir were stopped in June because they had already proven ineffective. However, the other trials continued.
The WHO's results appear to contradict a previous study from earlier this month, conducted by Gilead, which concluded that treatment with remdesivir cut Covid recovery time by five days compared to patients given a placebo. About 1,000 patients took part in that trial.
What has the reaction been?
Gilead Sciences Inc dismissed the findings.
"The emerging (WHO) data appears inconsistent, with more robust evidence from multiple randomised, controlled studies published in peer-reviewed journals validating the clinical benefit of remdesivir," the company said in a statement.
"We are concerned the data from this open-label global trial has not undergone the rigorous review required to allow for constructive scientific discussion, particularly given the limitations of the trial design."
But Prof Martin Landray, who runs the large trial Recovery in the UK, said the results of the trial were "important but sobering" - and added that there were already concerns about the cost and accessibility of remdesivir.
"Covid affects millions of people and their families around the world," he added.
"It is not a rare disease. We need scalable, affordable and equitable treatments. The WHO Solidarity trial has done the world a huge favour by producing clear, independent and robust results, showing once more the value of large randomised trials in providing the knowledge we need to tackle the worst consequences of the pandemic."
Remdesivir was given emergency use authorisation in the US from the country's Food and Drug Administration (FDA) on 1 May. Later that month it was approved for use in the UK, and has since been authorised for use in several other countries.
Remdesivir has been touted as a potential therapy since the beginning of the pandemic and gained greater attention when it formed part of Donald Trump's cocktail of treatments.
But the WHO trial, published online, gives a damning verdict.
There is a bit of uncertainty in the data, but the study says it "absolutely excludes" the idea remdesivir can save a significant number of lives and says the findings are "comfortably compatible" with the drug having no life-saving effect at all. It is a similar message for preventing people needing ventilation or speeding up people's recovery.
So far, doctors have been raiding the cupboard for existing drugs that can fight coronavirus.
The results have been disappointing with malaria drugs, HIV drugs, MS drugs and now an Ebola drug (remdesivir). Only an old steroid - dexamethasone - has proven life-saving.
The attention is now turning to new experimental therapies such as antibodies designed in the lab to fight the virus and new, untested, anti-viral drugs.
We are still waiting for the results of these trials, but the worry is "new" in medicine tends to mean "expensive", and that will raise questions about who gets to have them.
What about other Covid treatments?
Following the results of the Solidarity trial, Dr Swaminathan said the WHO was now "looking at what's next".
"We're looking at monoclonal antibodies, we're looking at immunomodulators and some of the newer anti-viral drugs that have been developed in the last few months," she said.
Meanwhile in China researchers say a vaccine under development has been found to be safe and to trigger immune responses in combined early and mid-stage trials.
However the researchers said it was not possible to say whether the antibody responses induced by the vaccine were sufficient to protect from infection because the trial was not designed to assess its efficacy.
Developed by the Beijing Institute of Biological Products, the vaccine has already been approved for an emergency inoculation programme in the country.