U.K. health regulators cleared a Covid-19 drug developed by Merck & Co. with partner Ridgeback Biotherapeutics LP, becoming the first country to authorize the pill that can be taken at home and has been shown to reduce death and hospitalization in higher-risk patients.

The Merck-Ridgeback drug will give the U.K. a much-needed weapon against Covid-19 as winter approaches.

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U.K. health regulators cleared a Covid-19 drug developed by Merck & Co. with partner Ridgeback Biotherapeutics LP, becoming the first country to authorize the pill that can be taken at home and has been shown to reduce death and hospitalization in higher-risk patients.

The Merck-Ridgeback drug will give the U.K. a much-needed weapon against Covid-19 as winter approaches.

A few other drugs have been found to reduce the risk of hospitalization, but they must be given intravenously. Doctors and patients have been waiting for an effective treatment that could be taken at home to prevent an infection from turning severe and requiring hospitalization.

“This will be a game-changer for the most vulnerable and the immunosuppressed, who will soon be able to receive the groundbreaking treatment,” U.K. Health Secretary Sajid Javid said in a statement.

Britain's Health Secretary Sajid Javid said the Merck pill was a ‘game-changer’ for the U.K.

Photo: daniel leal-olivas/Agence France-Presse/Getty Images

The U.S. and other countries are likely to follow suit authorizing the pill, molnupiravir, after a late-stage study recently found the drug reduced the risk of hospitalization or death by about 50%.

The U.S. Food and Drug Administration is reviewing molnupiravir. Experts who advise the agency are scheduled to meet in late November, which could set up the drug’s U.S. authorization by the end of the year.

Neither Merck nor the U.K. government said what the U.K. is paying for the drug, which has the brand name Lagevrio. The U.S. is paying $1.2 billion for 1.7 million courses of treatment, bringing a course of treatment to about $700.

Merck has said it plans to produce 10 million courses of the treatment by the end of this year, and at least 20 million in 2022.

The U.K. government last month said it secured 480,000 courses of treatment of molnupiravir.

The treatment becomes available as governments and public-health experts are cautioning there could be another virulent winter, especially among people who aren’t vaccinated.

“We will continue to move with both rigor and urgency to bring molnupiravir to patients around the world as quickly as possible,” said Merck Chief Executive Robert Davis.

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In the late-stage study, the drug was tested in unvaccinated people. The rate of side effects in study subjects who got molnupiravir and those who got a placebo was similar, according to the companies.

U.K. regulators cleared use of molnupiravir for high-risk adults with mild to moderate disease.

Antibody drugs, such as one from Regeneron Pharmaceuticals Inc., have also been authorized in the U.K., the U.S. and other countries to treat Covid-19 patients before they are hospitalized.

The antibody drugs have been found in clinical trials to be more effective than molnupiravir, but they require infusions at a doctor’s office, hospital or medical clinic.

One challenge with molnupiravir, doctors and health experts say, will be prescribing the drug to sick people early enough in the course of the disease for the drug to be most effective.

A course of treatment consists of taking a total of 40 pills—eight daily for five days—started within five days of showing symptoms.

Last week, Merck said it was licensing molnupiravir to United Nations-backed nonprofit the Medicines Patent Pool to provide more supplies to 105 low- and middle-income countries.

Merck also licensed the drug out to generic companies in India.

Many global patient advocates have expressed concerns that low- and middle-income countries won’t be able to get as much supply of molnupiravir as wealthier nations.

Kenilworth, N.J.-based Merck has said it estimates molnupiravir will generate sales of between $5 billion and $7 billion through 2022, including between $500 million and $1 billion this year.

Profit is split with Ridgeback, which had licensed the drug from Emory University, where it was invented.

Write to Jared S. Hopkins at jared.hopkins@wsj.com