Merck’s Covid pill is cleared for high-risk adults.

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

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 Covid treatment options authorized by the F.D.A. are “not accessible or clinically appropriate.”

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

The treatment — to be taken as 40 pills over five days — is expected to be available within a few weeks.

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 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 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.

Even before the F.D.A.’s decision, some doctors and health officials had tempered expectations for Merck’s drug. Studies indicate that it is far less effective than Pfizer’s version, 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 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, director of the Center for Pharmaceutical Policy and Prescribing at the University of Pittsburgh.

Merck has not yet released findings from company laboratory experiments on how well the drug can stop the Omicron variant from replicating. But the drug is designed to work in a way that makes scientists optimistic that it will work against Omicron.

The pills from Merck and Pfizer can be taken at home and are expected to reach many more people 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 options 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. Some hospitals have stopped using several of the antibody drugs because they are not likely to work against Omicron, and the only potent antibody treatment remaining against the variant is in very limited supply.

Merck’s treatment is expected to be in greater supply sooner than Pfizer’s. By the end of January, Merck is expected to make available to the federal government enough pills for 3.1 million people, at a cost of about $700 a person. The first 378,000 treatment courses are expected about two weeks after authorization.

Pfizer is expected to supply before the end of January enough of its product, known as Paxlovid, for 265,000 people in the United States. Initial supplies are expected in the next few days.

To get Merck’s pills, the F.D.A. said, patients will need to test positive for the virus and get a prescription from a health care provider, all no more than five days after symptoms start.

The F.D.A. did not specify which type of test would 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.