The U.S. Food and Drug Administration has authorized the emergency use of Merck’s antiviral pill for adults over the age of 18 who have tested positive for COVID-19 and are at high risk for severe disease. The decision comes a day after the agency authorized the emergency use of Pfizer’s antiviral pill for people 12 and older.

These drugs work by interfering with the virus’s genome as it multiples, creating mutations that disrupt replication and ultimately kill the virus. In clinical trials, Merck’s drug, called molnupiravir, was shown to reduce the risk of hospitalization and death by 30 percent. 

That is a drop in efficacy from Merck’s original announcement that people who took the pill were half as