The FDA said it should not be used as a substitute for other treatments.
The F.D.A. cleared the pill for adults who are vulnerable to becoming severely ill from Covid and for whom alternative treatment options are not accessible or clinically appropriate.
The agency believes that the pill is only modestly effective and may pose a risk to pregnant women.
The treatment is expected to be available early next month. The Omicron variant will be in greater supply in the coming weeks than other treatments in the United States.
Older people and people with certain diseases who can't get treatments like Pfizer's newly authorized pills or monoclonal antibody drugs would be able to get a prescription for the pills from Merck. Unvaccinated people will be eligible.
The agency authorized Pfizer's treatment for high-risk Covid patients as young as 12 years old, but only for adults because it may affect bone and cartilage growth.
Before the F.D.A. decision, some doctors and health officials had lowered expectations for the drug. It appears less effective than a pill from Pfizer, which received F.D.A. authorization on Wednesday, and is expected to become more widely available in the United States in a few months.
The drug reduced the risk of hospitalization or death by 30 percent when given to high-risk, unvaccinated people within five days of symptoms. The Pfizer pill reduced that risk by 88 percent.
Dr. Walid Gellad, director of the Center for Pharmaceutical Policy and Prescribing at the University of Pittsburgh, said that he doesn't think the new version of the drug will be a game-changer.
The drug was more effective against Omicron than it was against earlier versions of the virus, according to preliminary laboratory studies. The company and independent researchers have been running experiments on how well the drug can stop the Omicron variant from replicating.
The pills from the two companies can be taken at home and are expected to reach many more people around the world than the injections.
Until this week, the only authorized treatment option for Covid patients who are at high risk of becoming severely ill was the antibody drugs. The country has a shortage of the antibodies.
The federal government stopped distribution of treatments from Eli Lilly and Regeneron because it was unlikely they would work against Omicron. Almost all of the country's supply was represented by those treatments. There is only one treatment that is still potent against the new variant, and it is very limited.
Experts said that the value of the pill has increased.
Dr. Nahid Bhadelia, director of the Center for Emerging Infectious Diseases Policy and Research at Boston University, said that anything that keeps people out of the hospital has a role.
The rise in coronaviruses cases in the Midwest and the Northeast was caused by the Delta variant. The surge is being driven by Omicron, which became the dominant form of new cases in the United States.
40 pills are supposed to be taken over five days. The first 378,000 courses for the United States will be delivered within days. The full order of pills for 3.1 million people is expected to be fulfilled by the end of January, at a cost of $700 a person.
Paxlovid is expected to be available for 265,000 Americans by the end of January. The federal government allocated the first 65,000 treatment courses to states and said the first shipments would arrive within the next few days.
The federal government will distribute the pills to the states, which will then distribute them to health departments and pharmacies. The pills will be sent to community health centers.
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There are new treatments. The first two pill treatments for Covid-19 were authorized by the FDA. Some Covid patients who are at higher risk of becoming severely ill will be able to take the new drugs at home.
The F.D.A. said that patients must test positive for the virus and get a prescription from a health care provider no more than five days after symptoms emerge.
The F.D.A. didn't specify which type of test would be needed, opening the door for rapid tests that return results within 15 minutes. The 500 million rapid tests that President Biden plans to give out to the public may not be enough to meet the high demand.
Concerns about the risk of reproductive harm have been raised by the fact that the pill works by introducing errors into the virus's genes. The risk has not been proven in its studies.
The F.D.A. said that pregnant women should not take the pills. Women who may become pregnant should use contraception for at least four days after taking the pills, according to the agency. The agency said that the male partners of women who could become pregnant should use contraceptives for at least three months.
The pills could cause the virus to evolve in a way that does not stop it from replicating but instead leads to the emergence of a new variant, according to some scientists.
The pill is expected to be widely used in many parts of the world even if it plays a diminished role in the United States. John McGrath, an executive with the company, said on Thursday that they have 10 million treatment courses of the drug ready to ship.
Eight generics manufacturers in India will be able to make and sell the drug cheaper for use in poorer countries next year.
Regulators around the world have already approved use of the pill. More than 30 mostly wealthy countries are where the company has reached deals to sell the pill.
France canceled its order for the pills after its regulators declined to authorize the drug, citing concerns about its low efficacy and potential safety risks.
In October, the company said it expected the pill to bring in $7 billion in revenue by the end of the year. The company would split profits equally with Ridgeback Biotherapeutics, a small Miami firm that has helped shepherd the drug's development.
