In the trial, lead by a team from Oxford University, around 2,000 hospital patients were given dexamethasone and were compared with more than 4,000 who did not get the drug.
For patients on ventilators it cut death risk from 40% to 28%. For patients needing oxygen it cut death risk 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.”