The Food and Drug Administration on Thursday authorized Merck’s COVID-19 pill for certain adults.

The authorization for molnupiravir is limited to adults over 18 who have a high risk of severe illness and “for whom alternative FDA-authorized treatment options are not accessible or medically appropriate,” the FDA said in a press release Thursday. It’s not recommended for use in pregnant people.

This is the second COVID-19 treatment in pill form after Pfizer’s pill, Paxlovid, was authorized Wednesday. Paxlovid is authorized for a slightly larger pool of people — anyone over 12 and at high risk of severe illness.

“As new variants of the virus continue to emerge, it is crucial to expand the country’s arsenal of COVID-19 therapies using emergency use authorization, while continuing to generate additional data on their safety and effectiveness,” Dr. Patrizia Cavazzoni, director of the FDA’s Center for Drug Evaluation and Research, said of the Merck pill’s authorization in a statement on Thursday.

Merck’s treatment was found to be less effective than Pfizer’s in studies, with a 30% reduction of severe illness or death as opposed to an 89% reduction of the same outcome with Pfizer’s pill.

Still, both have been heralded as significant therapeutic steps in the fight against COVID.

“This approval offers one more line of defense as we deal with a winter surge. While Molnupiravir did not show the same high efficacy as Paxlovid, any additional help for high risk patients is welcome as we expect high demand for at-home treatment options,” said Dr. John Brownstein, an epidemiologist at Boston Children’s Hospital and ABC News contributor.

The US government has purchased 10 million Pfizer treatment courses, more than any other country in the world, and 3 million Merck courses, White House COVID coordinator Jeff Zients said on Wednesday.

For Pfizer, 265,000 doses will be available in January. Production will ramp up over time, the result of a complex active ingredient in the pill that takes time to make, and all 10 million will be delivered by late summer.

“The very good news is that we have a product now that has about a 90% efficacy in preventing hospitalization. But the sobering news is unfortunately, it is really a quite complicated and complex synthetic process,” Dr. Anthony Fauci, chief medical advisor to the president, said on Wednesday after Pfizer’s pill was authorized by the FDA.

He added that they will be working with the company and “hopefully we’ll be able to shorten that process.”

For Merck, all three million treatment courses are expected to be delivered in January.

Both Pfizer and Merck’s pills should be taken early, within 3-5 days of getting COVID, and require several pills, multiple times a day for five days.

ABC News’ Sony Salzman contributed to this report.