For the second time in as many days, the Food and Drug Administration (FDA) has approved a COVID-19 pill. Thursday’s approval was for Merck’s COVID-19 pill, also known as Molnupiravir. This followed the FDA approving the Pfizer COVID-19 pill, also known as Paxlovid, on Wednesday.

Unlike the Pfizer pill, which is approved for adults and kids 12 and up who have recently tested positive for COVID-19, the Merck pill is only approved for adults. According to an FDA news release, “Molnupiravir is not authorized for use in patients younger than 18 years of age because Molnupiravir may affect bone and cartilage growth.”

The Merck pill also carries a warning regarding use during pregnancy. The FDA said women of childbearing age should use birth control during treatment and for a few days after. Men should use birth control for at least three months after their final dose.

“Based on findings from animal reproduction studies, Molnupiravir may cause fetal harm when administered to pregnant individuals,” the FDA said.

For both pills, the person seeking them must be “at high risk for progression to severe COVID-19, including hospitalization or death.” However, there is one other difference.

“Molnupiravir is limited to situations where other FDA-authorized treatments for COVID-19 are inaccessible or are not clinically appropriate and will be a useful treatment option for some patients with COVID-19 at high risk of hospitalization or death,” FDA Center for Drug Evaluation and Research Director Dr. Patrizia Cavazzoni said in a statement. Her statement all but confirms that the Pfizer pill will become the first-choice oral treatment against COVID-19.

Part of the reason for this is superior benefits in the Pfizer COVID-19 pill. Research released last month found Paxlovid caused an “89% reduction in risk of COVID-19-related hospitalization or death from any cause compared to placebo.” Meanwhile, initial research found the Merck pill only reduced hospitalizations and deaths by half. That effectiveness was eventually dropped to 30% after additional data was released.

Federal officials have agreed to purchase enough of the Merck COVID-19 pill to treat 3.1 million people. Merck officials say several hundred thousand treatment courses will be available in the U.S. in the next several days and a million will be available over the next few weeks.