WASHINGTON — A panel of U.S. health advisers on Tuesday narrowly backed a closely watched COVID-19 pill from Merck, setting the stage for a likely authorization of the first drug that Americans could take at home to treat the coronavirus.

The Food and Drug Administration panel voted 13-10 that the antiviral drug’s benefits outweigh its risks, including potential birth defects if used during pregnancy.

“I see this as an incredibly difficult decision with many more questions than answers,” said panel chairman Dr. Lindsey Baden of Harvard Medical School, who voted in favor of the drug. He said the FDA would have to carefully tailor the drug’s use for patients who stand to benefit most.

The recommendation came after hours of debate about the drug’s modest benefits and potential safety issues.

Most experts backing the treatment stressed that it should not be used by anyone who is pregnant and called on FDA to recommend extra precautions before the drug is prescribed, such as pregnancy tests for women of child-bearing age.

The vote backed the drug for adults with mild-to-moderate COVID-19 who face the greatest risks, including older people and those with conditions like obesity and asthma.

Most experts also said the drug shouldn’t be used in vaccinated people, who weren’t part of Merck’s research.

The FDA isn’t bound by the recommendation and is expected to make its own decision before year’s end.

The drug, molnupiravir, could provide a much-needed weapon against the virus as colder weather pushes case counts higher and the U.S. braces for the arrival of the new omicron variant.

Merck hasn’t specifically tested its drug against the new variant but said it should have some potency based on its effectiveness against other strains.

That uncertainty frustrated panelists as they grappled with whether to back the treatment for Americans. “With no data saying it works with new variants, I really think we need to be careful about saying that this is the way to go,” said Dr. David Hardy of Charles Drew University School of Medicine and Science, who voted to back the treatment.