A cheap and widely available drug can help save the lives of patients seriously ill with coronavirus.
The low-dose steroid treatment dexamethasone is a major breakthrough in the fight against the deadly virus, UK experts say.
The drug is part of the world's biggest trial testing existing treatments to see if they also work for coronavirus.
It cut the risk of death by a third for patients on ventilators. For those on oxygen, it cut deaths by a fifth.
Had the drug had been used to treat patients in the UK from the start of the pandemic, up to 5,000 lives could have been saved, researchers say.
And it could be of huge benefit in poorer countries with high numbers of Covid-19 patients.
The UK government has 200,000 courses of the drug in its stockpile and says the NHS will make dexamethasone available to patients.
Prime Minister Boris Johnson said there was a genuine case to celebrate "a remarkable British scientific achievement", adding: "We have taken steps to ensure we have enough supplies, even in the event of a second peak."
Chief Medical Officer for England Prof Chris Whitty said it would save lives around the world.
About 19 out of 20 patients with coronavirus recover without being admitted to hospital.
Of those who are admitted, most also recover but some may need oxygen or mechanical ventilation.
And these are the high-risk patients dexamethasone appears to help.
The drug is already used to reduce inflammation in a range of other conditions, including arthritis, asthma and some skin conditions.
And it appears to help stop some of the damage that can happen when the body's immune system goes into overdrive as it tries to fight off coronavirus.
This over-reaction, a cytokine storm, can be deadly.
In the trial, led by a team from Oxford University, about 2,000 hospital patients were given dexamethasone and compared with more than 4,000 who were not.
For patients on ventilators, it cut the risk of death from 40% to 28%.
For patients needing oxygen, it cut the risk of death from 25% to 20%.
Chief investigator Prof Peter Horby said: "This is the only drug so far that has been shown to reduce mortality - and it reduces it significantly. It's a major breakthrough."
Lead researcher Prof Martin Landray said the findings suggested one life could be saved for:
- every eight patients on a ventilator
- every 20-25 treated with oxygen
"There is a clear, clear benefit," he said.
"The treatment is up to 10 days of dexamethasone and it costs about £5 per patient.
"So essentially it costs £35 to save a life.
"This is a drug that is globally available."
When appropriate, hospital patients should now be given it without delay, Prof Landray said.
But people should not go out and buy it to take at home.
Dexamethasone does not appear to help people with milder symptoms of coronavirus who do not need help with their breathing.