U.S. regulators on Thursday issued an emergency use authorization for an antiviral COVID-19 pill developed by American pharmaceutical firm Merck & Co., giving the country its second such at-home treatment option in the fight against the coronavirus pandemic.

The treatment, called molnupiravir, is not recommended during pregnancy due to concerns over birth defects. Its use is limited compared with Pfizer Inc.'s pill that was authorized a day before, although both have been approved for mild-to-moderate coronavirus cases in people at high risk of hospitalization or death.

Supplied photo shows the oral COVID-19 medicine molnupiravir. (Photo courtesy of Merck & Co.)(Kyodo)

The U.S. Food and Drug Administration emphasized that molnupiravir is for situations in which other authorized treatments for COVID-19 are inaccessible or clinically inappropriate.

Molnupiravir, greenlit by British regulators in November, has drawn attention as the world's first approved antiviral COVID-19 treatment which can be taken orally. The Japanese arm of Merck applied in early December for approval to produce and sell the drug in the country.

It is a medication that works by introducing "errors" into the virus' genetic code, thereby preventing the virus from further replication, according to the FDA. It is meant to be taken as soon as possible after a COVID-19 diagnosis and within five days of symptom onset.

Based on findings from animal studies, molnupiravir may cause fetal harm when administered to pregnant individuals. Both men and women capable of conception are advised to use birth control beyond the course of treatment, with women recommended to do so for four days after the final dose and men for at least three months after the last dose.

The treatment is not authorized for use in patients younger than 18 years of age because it may affect bone and cartilage growth, the FDA said.

Clinical trial data showed that molnupiravir reduced the risk of hospitalization or death by 30 percent.

Meanwhile, Pfizer's treatment Paxlovid was authorized for patients aged 12 or older. The experimental pill showed nearly 90 percent efficacy in preventing hospitalization or death in high-risk patients, according to final study results.

People with diabetes or heart disease are among those considered to be at high risk of getting seriously ill from COVID-19.

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