The U.S. health regulator approved a second pill for COVID-19 on Thursday, offering another easy-to-use drug to combat the rising trend of omicron infections.

The US Food and Drug Administration’s announcement about Merck’s molnupiravir came a day after the agency cleared Pfizer from a competing drug.

Pfizer’s pill Paxlovid is likely to become the preferred treatment for fighting the virus because of its superior benefits and milder side effects.

Therefore, it is expected that Merck’s pill will be less effective in fighting the pandemic than predicted a few weeks ago. Its ability to stop severe COVID-19 is much smaller than initially announced, and the drug label will warn of serious safety issues, including potential birth defects.

After being purchased by the federal government, both of these treatments will be provided to patients in the United States for free.

The FDA authorizes Merck’s drugs to be used in adults who test positive for COVID-19, have early symptoms, and are at the highest risk of hospitalization, including the elderly and people with diseases such as obesity and heart disease.

The agency stated that for “patients who cannot obtain FDA-approved alternative treatments for COVID-19 or who are clinically inappropriate”, monelavir should be considered. It does not include restrictions on Paxlovid, which are authorized for patients 12 years and older.

Molnupiravir manufactured by Merck and Ridgeback Biotherapeutics will also warn against use during pregnancy. The FDA stated that women of childbearing age should use contraception during treatment and within a few days after treatment, while men should use contraception for at least three months after the last dose. The federal agency also stated that monopiravir should not be used in patients under the age of 18 because it may affect bone growth.

Dr. Nick Kartsonis, senior vice president of clinical research at Merck, said that company scientists are still working on the drug and they hope to eventually get approval for use in children.

Last month, the FDA advisory panel only reluctantly approved the drug, warning that its use must be strictly targeted at the patients who would benefit the most, so these restrictions are expected to be introduced.

Kartsonis said that Merck’s researchers are “very satisfied” with the safety of its drug.

He also pointed out that molnupiravir has been studied in more than 1,000 people, and studies have shown that only a small percentage of patients have side effects such as diarrhea and nausea.

“In fact, the tolerability of this drug looks very good,” he said.

Pfizer’s Paxlovid also has limitations. Regulators have stated that doctors must consider the interaction between it and other drugs that patients may be taking, and they point out that it should not be prescribed to patients with severe liver or kidney problems.

These two treatments should also be started within five days of the onset of symptoms. This period will put pressure on patients and require them to be tested and diagnosed quickly.

The effects of these two pills are different, and Pfizer’s risks are also different. In addition, Pfizer’s drug is about three times more efficient in testing, reducing the hospitalization rate and mortality of high-risk patients by nearly 90%, while Merck’s is 30%.

Some experts question whether Merck’s drugs will play a big role in the U.S.

“With a sufficient supply of Pfizer’s pills, I don’t think it will be used,” said Dr. Gregory Poland of the Mayo Clinic, referring to Merck’s drugs. “There is no reason, because it is less effective and has a higher risk of side effects.”

Currently, even if health officials are prepared to respond to record cases, hospitalizations, and deaths caused by the omicron variant, the FDA’s decision provides another possible option to combat the virus that has killed more than 800,000 Americans.

According to data collected by Johns Hopkins University, the 7-day rolling average of COVID-19 cases in the United States climbed to more than 160,000 on Wednesday. This is more than twice the average at the end of November.

Antiviral drugs such as Merck and Pfizer are expected to be effective against omicron because they do not target the spike protein where most of the variants have worrying mutations.

Researchers warn that older COVID-19 treatments given intravenously are unlikely to be effective against new strains.

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The FDA made a decision based on Merck’s decision. The results showed that nearly 7% of patients taking the drug were eventually admitted to the hospital, and one person died at the end of 30 days. In contrast, 10% of hospitalized patients taking placebo and 9 died.

Merck said it will provide hundreds of thousands of treatment courses in the United States in the next few days and one million treatment courses in the next few weeks. Pfizer’s drug supply will be more limited before it increases in 2022.

The United States will pay approximately US$700 for each course of Merck’s treatment, which requires patients to take four pills twice a day within five days. An evaluation by Harvard University and King’s College London estimated that the cost of a course of 40 pills is about $18.

Merck’s drug inserted a small error in the genetic code of the coronavirus to slow its reproduction. This genetic effect has raised concerns that the drug may cause mutations in human fetuses and even stimulate the production of more virulent virus strains. But FDA scientists say that the risk of mutation is largely theoretical because people take the drug for a short period of time.