Pfizer’s is the second pill to show effectiveness against Covid-19, and it is the first purpose-built to attack the virus that causes the disease.

Pfizer announced on Friday that its pill to treat Covid-19 had been found in a key clinical trial to be highly effective at preventing severe illness among at-risk people who received the drug soon after they exhibited symptoms, making it the second antiviral pill to demonstrate efficacy against Covid.

The drug appears to be more effective than a similar offering from Merck, which is awaiting federal authorization. Pfizer’s pill, which will be sold under the brand name Paxlovid, cut the risk of hospitalization or death by 89 percent when given within three days after the start of symptoms.

Pfizer said an independent board of experts monitoring its clinical trial recommended that the study be stopped early because the drug’s benefit to patients had proved so convincing. The company said it planned to submit the data as soon as possible to the Food and Drug Administration to seek authorization for the pill to be used in the United States.

“The results are really beyond our wildest dreams,” said Annaliesa Anderson, a Pfizer executive who led the drug’s development. She expressed hope that Paxlovid “can have a big impact on helping all our lives go back to normal again and seeing the end of the pandemic.”

The arrival of a new class of easy-to-use pills that dramatically reduce hospitalizations could help bring the curtain down on the most severe phase of the pandemic, at least in wealthy countries where most adults have been vaccinated.

The Pfizer and Merck pills, which can be dispensed at pharmacies and taken at home, are expected to reach many more people than monoclonal antibody treatments, which are usually given by intravenous infusion at a clinic.

The Pfizer treatment could become available in the next few months, though supplies are likely to be limited at first. The Pfizer and Merck pills are both geared toward patients regarded as high-risk, such as those above age 60 or with conditions like obesity that make them more susceptible to severe consequences from Covid.

President Biden said on Friday that the pill would be “another tool in our toolbox to protect people from the worst outcomes of Covid,” but he emphasized that the best approach was to prevent infections through vaccination.

Pfizer said it expects to be able to produce enough pills for more than 180,000 people by the end of this year and for more than 21 million people in the first half of next year. Merck, too, has said it plans to ramp up production over the next year.

The U.S. government has been in negotiations with Pfizer for enough pills for 1.7 million courses of treatment, with an additional option for 3.3 million, according to a senior administration official. That is about the same quantity that the United States has ordered from Merck. The government expects to pay about $700 per treatment course for both drugs, the official said.

A number of wealthy countries, including Britain and Australia, have also raced to lock up supplies of Pfizer’s drug.

Pfizer said it planned to offer poorer countries the drug at discounted prices. The company has been in talks with a United Nations-backed nonprofit, the Medicines Patent Pool, to allow the pill to be made and sold inexpensively in such countries; Merck has already reached a similar deal.

The treatment consists of 30 pills given over five days. That includes 10 pills of ritonavir, an old H.I.V. drug, which helps Pfizer’s drug remain active in the body longer. (Merck’s treatment course is 40 pills over five days.)

The pills so far have mainly been tested in high-risk patients. But Pfizer is also running trials on low-risk patients and people in the same household as those infected with the virus.

The efficacy results announced on Tuesday included data from more than 1,200 adults in the United States and overseas who received either Pfizer’s drug or a placebo pill after contracting Covid. The volunteers were enrolled between July and September, when the Delta variant was ripping across the globe. They were unvaccinated and had at least one characteristic that put them at greater risk of becoming severely ill from the virus, such as older age or having obesity or diabetes.

Pfizer’s 89 percent efficacy figure came from the group of volunteers who started treatment within three days of developing symptoms. Including people who began treatment on the fourth or fifth day, the pill reduced the risk of hospitalization or death by 85 percent.

By contrast, the Merck pill was about 50 percent effective when given within five days of the onset of symptoms, though the different designs and timing of the Pfizer and Merck trials make such comparisons imprecise. Monoclonal antibody treatments reduce hospitalizations and deaths by at least 70 percent in high-risk Covid patients, but those treatments are more expensive and more cumbersome to administer.

Study volunteers who got the Pfizer pill reported mostly mild side effects at a slightly lower rate than those who received the placebo pill. That was a promising sign for the drug’s safety, indicating that Covid symptoms are probably more bothersome than any of the pill’s side effects.

The origins of Pfizer’s pill stretch back 19 years, to the SARS epidemic. Early last year, Pfizer began modifying the drug’s design so that it could be used to fight Covid and taken as a pill rather than intravenously.

Pfizer’s drug is in the class of so-called protease inhibitors that are commonly used to treat H.I.V. and hepatitis C. The drug is designed to stop the coronavirus from replicating by blocking the activity of a key enzyme that the coronavirus uses to replicate inside cells.

Pfizer also said that its studies showed that the drug was safe and did not cause worrisome mutations. Some scientists have raised that concern about Merck’s pill, which works by inserting errors into the virus’s genetic code to stop it from replicating. Pfizer’s pill doesn’t do that.

Britain, which on Thursday became the first government to authorize Merck’s pill, recommended that it not be used in women who are pregnant, breastfeeding or who could become pregnant during treatment and for four days after.