Publisher Health

The Food and Drug Administration on Thursday authorized a second antiviral pill for Covid but said it should not be preferred over other treatments.

The F.D.A. cleared the pill, developed by Merck and known as molnupiravir, 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’s decision reflects concerns that Merck’s pill is only modestly effective and carries possible safety risks, including for pregnant women.

Merck’s treatment is expected to be available early next month. With the Omicron variant driving an onslaught of infections, the drug will be in greater supply in the coming weeks than other treatments in the United States.

Older people and those who have conditions like obesity, diabetes or heart disease would be eligible to get a prescription for Merck’s pills if they become sick from the coronavirus and cannot get treatments like Pfizer’s newly authorized pills or monoclonal antibody drugs. Both vaccinated and unvaccinated people will be eligible.

But while the agency authorized Pfizer’s treatment for high-risk Covid patients as young as 12, it said that it was authorizing Merck’s drug only for adults because it “may affect bone and cartilage growth.”

Even before the F.D.A.’s decision, some doctors and health officials had tempered expectations for Merck’s drug. It appears far less effective than a similar pill from Pfizer, which received F.D.A. authorization on Wednesday and is expected to become more widely available in the United States starting in a few months.

In a key clinical trial, Merck’s drug reduced the risk of hospitalization or death by 30 percent when given to high-risk, unvaccinated people within five days of the onset of symptoms. Pfizer’s pill was found to reduce that risk by 88 percent.

“I don’t think Merck’s version is going to be the game changer,” said Dr. Walid Gellad, who directs the Center for Pharmaceutical Policy and Prescribing at the University of Pittsburgh.

Dr. Dean Li, a Merck executive, said on Thursday that preliminary laboratory studies suggested that the drug was equally effective against Omicron as it was against earlier versions of the virus. The company and independent researchers have been running experiments on how well the drug can stop the Omicron variant from replicating.

The pills from Merck and Pfizer can be taken at home and are expected to reach many more people globally than monoclonal antibody treatments, which are typically given via intravenous infusions by a health care provider.

Until this week, the antibody drugs have been the only authorized treatment option for Covid patients who are at high risk of becoming severely ill. But Omicron has wreaked havoc on the country’s supply of the antibodies.

On Thursday, the federal government paused distribution of the treatments from Regeneron and Eli Lilly, saying it was unlikely they would work against Omicron. Those treatments represented nearly all of the country’s supply. The only antibody treatment that remains potent against the new variant, from GlaxoSmithKline and Vir Biotechnology, is in very limited supply for now.

In that context, the value of Merck’s pill has increased, experts said.

“Anything that keeps people out of the hospital, even incremental, has a role,” said Dr. Nahid Bhadelia, director of the Center for Emerging Infectious Diseases Policy and Research at Boston University.

In early November, the Delta variant started fueling a rise in coronavirus cases, particularly in the Midwest and the Northeast. Omicron, which rapidly became the dominant form of new cases in the United States, is now driving the surge.

Merck’s treatment is meant to be taken as 40 pills over five days. The company is ready to ship the first 378,000 courses for the United States within days. Merck is expected to fulfill the federal government’s full order of enough pills for 3.1 million people, at a cost of about $700 a person, by the end of January.

Pfizer is expected to provide enough of its pills, known as Paxlovid, for 265,000 Americans before the end of January. The federal government on Thursday allocated the first 65,000 treatment courses to states and said the first shipments would begin arriving within the next few days.

As was done with Covid vaccines, the federal government will allocate the pills to states, which can then distribute them to health departments and pharmacies. The government will also ship the pills directly to community health centers.

To get Merck’s pills, the F.D.A. said, 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. did not specify which type of test will be needed, opening the door for widespread use of over-the-counter rapid antigen tests, which return results within 15 minutes. President Biden announced plans this week to buy 500 million rapid tests to distribute free to the public, but that may not be sufficient to meet what is expected to be very high demand.

Merck’s pill works by introducing errors into the virus’s genes to stop it from replicating, which has raised concerns about the risk that it could cause reproductive harm. The risk is hypothetical, and Merck says it has not been borne out in its studies.

The F.D.A. said that women who were pregnant should generally not take the pills, but that there could be exceptions. The agency said that women who may become pregnant should use contraception while taking the pills and for at least four days after. The male partners of women who could become pregnant should use contraception while taking the pills and for at least three months after, the agency said.

Some scientists have also raised concerns that the pills could in theory cause the virus to mutate in a way that does not stop it from replicating but instead leads to the emergence of a new variant.

Even if it plays a diminished role in the United States, Merck’s pill is expected to be widely used in many parts of the world. Merck has 10 million treatment courses of the drug ready to ship and expects to produce at least 20 million more in 2022, John McGrath, a Merck executive, said on Thursday.

Additional production next year will come from eight generics manufacturers in India as well as other producers that will be able to make and sell the drug inexpensively for use in poorer countries.

Several regulators around the world, including in Britain and Denmark, have already greenlighted use of Merck’s pill. The company has reached deals to sell the pill directly to more than 30 mostly wealthy countries.

But this week, one of those buyers, France, said it was canceling its order for the pills after its regulator declined to authorize the drug, citing concerns about its low efficacy and potential safety risks.

Merck said in October that it expected the pill to generate up to $7 billion in revenue globally through the end of 2022. The company would split profits equally with Ridgeback Biotherapeutics, a small Miami firm that has helped shepherd the drug’s development.