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Based on the data in the company’s release, the drug appears to have a clean safety profile, meaning there were no serious side effects in trial volunteers.

Because molnupiravir works by disrupting how the coronavirus replicates RNA, there could be a concern of a similar effect on human DNA or RNA. Merck reportedly has data from laboratory studies indicating that molnupiravir does not cause mutations in humans, but “the FDA will obviously need to see and evaluate this safety data in the approval process,” Dr. Shaw says.  

Dr. Shaw notes that several approved antiviral drugs already in wide clinical use—such as acyclovir and related drugs for herpesvirus infections, and reverse-transcriptase inhibitors for HIV infection—also work (via different mechanisms) in interfering with the replication of viral DNA or RNA.

Yale Medicine infectious diseases specialist Jaimie Meyer, MD, MS, noted that in its clinical trial, Merck didn’t test the drug on pregnant women. “In the trial, not only did they exclude women who were pregnant, breastfeeding, or anticipating becoming pregnant, but they also told the men enrolled in the trial that they couldn’t have unprotected sex with women for a week after they were done with their medication,” she says.

The concern might be that this drug would interfere with RNA replication needed for fetus development and cause birth defects. This will be important to tease out as this drug moves from clinical trials to the market, she adds.