The first pill designed to treat symptomatic Covid-19 has been approved by the UK medicines regulator.
The tablet - molnupiravir - will be given twice a day to vulnerable patients recently diagnosed with the disease.
In clinical trials the pill, originally developed to treat flu, cut the risk of hospitalization or death by about half.
Health Secretary Sajid Javid said the treatment was a "gamechanger" for the most frail and immunosuppressed.
First oral treatment
Molnupiravir, developed by the US drug companies Merck, Sharp and Dohme (MSD) and Ridgeback Biotherapeutics, is the first antiviral medication for Covid-19 which can be taken as a pill rather than injected or given intravenously.
The UK has agreed to purchase 480,000 courses with the first deliveries expected in November.
Initially it will be given to both vaccinated and unvaccinated patients through a national study, with extra data on its effectiveness collected before any decision to order more.
The drug needs to be given within five days of symptoms developing to be most effective.
The new treatment targets an enzyme that the virus uses to make copies of itself, introducing errors into its genetic code. That should prevent it from multiplying, so keeping virus levels low in the body and reducing the severity of the disease.
Merck said that approach should make the treatment equally effective against new variants of the virus as it evolves in the future.
The UK regulator, the MHRA, said the tablet had been authorized for use in people who have mild to moderate Covid-19 and at least one risk factor for developing severe illness such as obesity, old age, diabetes or heart disease.
Earlier clinical trials of molnupiravir on 775 patients who had recently caught Covid-19 found:
7.3% of those given the drug were hospitalized
that compares with 14.1% of patients who were given a placebo or dummy pill
there were no deaths in the molnupiravir group, but eight patients who were given a placebo in the trial later died of Covid-19
The results were published in a press release and have not yet been peer-reviewed.
But data suggest molnupiravir needs to be taken soon after symptoms develop to have an effect.
In its approval document, the MHRA recommends the drug is used "as soon as possible" following a positive Covid-19 test and within five days of symptoms onset.
The UK government has not disclosed how much its initial contract for 480,000 courses of molnupiravir is worth. But US authorities recently made an advance purchase of 1.7 million courses at a cost of roughly $1.2 billion, or $700 for each patient.
Merck is the first company to report trial results of a pill to treat Covid-19, but other companies are working on similar treatments.
Its US rival Pfizer has started trials of two different antiviral tablets, while Swiss company Roche is working on a similar medication.