An Antiviral ‘Covid-19 Pill’ Shows Promise for Treating Infection

Everything you need to know about the experimental drug molnupiravir

Emily Mullin
Medium Coronavirus Blog
3 min readMar 11, 2021


Photo: Christina Victoria Craft/Unsplash

Vaccines are being rolled out across the country, but many are still at risk of infection with Covid-19. Though there are no drugs available to take at home yet, a pill for the coronavirus may be on the horizon.

An experimental drug meant to reduce the length of illness is showing promise in a small trial. If it works, doctors could soon prescribe it to people who get sick with the coronavirus, like they do with Tamiflu for seasonal flu.

The pill, which is being developed by Miami-based biotech firm Ridgeback Biotherapeutics and the drug giant Merck, is an antiviral called molnupiravir. Elizabeth Duke, MD, research associate in the Vaccine and Infectious Disease Division at the Fred Hutchinson Cancer Research Center in Seattle, tells the Coronavirus Blog that the drug works by stopping the virus from making more copies of itself.

“It won’t protect cells that are already infected, but it will prevent those infected cells from making new copies of the virus that might infect other cells,” says Duke, who is principal investigator at Fred Hutch for Ridgeback Biotherapeutics’ molnupiravir clinical trial.

In preliminary results announced on March 6, the drug was safe and significantly reduced infectious virus in 182 adults with Covid-19 after a five-day course of the antiviral.

“By day five, there was no infectious virus left in the nostrils of the people who had been given the treatment,” Duke says. Meanwhile, 24% of people who received a placebo tested positive for the virus after five days.

The findings don’t mean that the drug is a cure. The study isn’t finished yet, and it’s still unknown whether the drug makes symptoms less severe or shortens the recovery time from Covid-19.

If shown to be effective, the drug could play an important role in treating people who get sick with Covid-19 but aren’t so ill that they need to be hospitalized. Just one antiviral, remdesivir, has been authorized to treat Covid-19. But because it’s delivered intravenously, remdesivir is only available to hospitalized patients with moderate or severe disease. It’s been shown to provide a modest benefit by reducing the length of a person’s hospital stay.

Molnupiravir was initially discovered at Emory University, where researchers originally planned to study it for flu. When the pandemic hit, they switched gears and began testing it against SARS-CoV-2. In petri dishes, the drug successfully stopped the virus from reproducing. Ridgeback Biotherapeutics acquired the drug in March 2020, and two months later, the company sold the drug rights to Merck.

Intriguingly, the drug has shown potent activity against a wide range of other viruses in the lab, including SARS, MERS, and chikungunya. It works by attacking a site on the virus that helps it replicate.

To get the most benefit from an antiviral like molnupiravir, scientists think it needs to be given as early as possible after symptoms begin.

“In general, we think the best time to treat someone is early in disease, because all an antiviral can do is stop the virus. It can’t reverse damage that has been done to tissue,” Duke says. That’s been a major challenge with treating Covid-19 because the virus can ravage the lungs. “Once people’s lungs have become really inflamed, an antiviral isn’t going to calm that inflammation down. It’s just going to make the virus stop replicating.”

More data from the molnupiravir trial should be released in the coming weeks. After that, the drug will need to be tested in a larger trial, likely with a few thousand people, to determine a clear benefit.



Emily Mullin
Medium Coronavirus Blog

Former staff writer at Medium, where I covered biotech, genetics, and Covid-19 for OneZero, Future Human, Elemental, and the Coronavirus Blog.