Some laboratory studies suggest that molnupiravir can insert errors in DNA, which could in theory harm a developing fetus, sperm cells or children.

By Benjamin Mueller

A new Covid-19 pill from Merck has raised hopes that it could transform the landscape of treatment options for Americans at high risk of severe disease at a time when the Omicron variant of the coronavirus is driving a surge of cases in highly vaccinated European countries.

But two weeks after a Food and Drug Administration expert committee narrowly voted to recommend authorizing the drug, known as molnupiravir, the F.D.A. is still weighing Merck’s application. Among the biggest questions facing regulators is whether the drug, in the course of wreaking havoc on the virus’s genes, also has the potential to cause mutations in human DNA.

Scientists are especially worried about pregnant women, they said, because the drug could affect a fetus’s dividing cells, theoretically causing birth defects. Members of the F.D.A. expert committee expressed those same concerns during a public meeting on Nov. 30.

“Do we want to reduce the risk for the mother by 30 percent while exposing the embryo and the fetus to a much higher risk of harm by this drug?” Dr. James Hildreth, the president of Meharry Medical College in Tennessee, said at the meeting. “My answer is no, and there is no circumstance in which I would advise a pregnant woman to take this drug.”

The F.D.A. advisers also noted that the risks could extend to other patients, including men wanting to become fathers, though those risks remain poorly understood and Merck said its own studies had turned up no evidence that the drug causes DNA mutations.

Crucially, molnupiravir is expected to work against Omicron. But it has drawn concern from some scientists and regulators in Europe for being less effective than certain other treatments: It has been shown to reduce the risk of hospitalization and death by 30 percent if given within five days of symptoms emerging.

Here’s what scientists know about how the drug works and its potential risks.

When the drug is processed in the body, it creates compounds that closely resemble one of the building blocks of RNA, the genetic material inside the coronavirus.

That causes problems for the coronavirus as it makes copies of itself: Once the virus enters a cell and starts replicating, the drug compound can slip into the virus’s RNA and insert enough errors that the virus cannot survive.

“What molnupiravir does is it disguises itself,” Elizabeth Campbell, an expert in structural biology at the Rockefeller University who studies coronavirus antivirals, said in an interview. “It can propagate errors that are going to be sprinkled all over the genome.”

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