The US on Thursday authorised Merck & Co's antiviral pill for Covid-19 for certain high-risk adult patients, a day after giving a broader go-ahead to a similar but more effective treatment from Pfizer Inc.

The US Food and Drug Administration (FDA) said Merck's drug, which is designed to introduce errors into the genetic code of the virus, could be used when other authorised treatments are not accessible or clinically appropriate.

The drug, molnupiravir, was developed with Ridgeback Biotherapeutics and shown to reduce hospitalisations and deaths by around 30 per cent in a clinical trial of high-risk individuals early in the course of the illness.

The authorisation allows the use of the drug for mild-to-moderate Covid-19 and along with the Pfizer pill, it could be an important tool against the fast-spreading Omicron variant, which is now dominant in the US.

Pfizer's drug, Paxlovid, was authorised on Wednesday for people aged 12 and older and has shown to be nearly 90pc effective in preventing hospitalisations and deaths in patients at high risk of severe illness, according to trial data.

The FDA says some patients should avoid Pfizer two-drug regimen because it includes an older antiviral called ritonavir that is known to have interactions with some other prescription medicines. It is also not recommended for people with severe kidney issues.

Merck plans to ship hundreds of thousands of treatment courses in coming days and one million over the next few weeks. Pfizer plans to ship about 250,000 courses in the next month.

Availability of other treatments is the first consideration for doctors thinking of prescribing Merck's treatment, according to Patrizia Cavazzoni, the director of FDA's Centre for Drug Evaluation and Research.

The FDA has authorised intravenous treatments, mostly for hospital use, called monoclonal antibodies but availability is limited and efficacy against the Omicron variant is lower.

FDA official John Farley said during a press briefing that GlaxoSmithKline's and Vir Biotech's antibody drug — which has been shown to work against Omicron — is expected to be in short supply this winter.

Merck said that its treatment had advantages over the Pfizer pill.

“It doesn't require any second drug to boost its efficacy, and you can give it in a variety of special patient populations, including people who have significant issues with liver function or kidney function,” Nick Kartsonis, Merck's senior vice president of clinical research for vaccines and infectious diseases, told Reuters.

Read: The era of anti-Covid pills begins

Merck's drug is not authorised for use in patients younger than 18 because molnupiravir may affect bone and cartilage growth, the FDA said. The pill is not recommended for use during pregnancy, the agency added.

The agency advised that men of reproductive potential use a reliable method of birth control during treatment with molnupiravir, and for at least three months after the final dose.

The drug is meant to be taken twice a day — four pills each time — for five days, making a full treatment course of 40 pills.

The treatment was priced at about $700 per course in a deal with the US government for up to 5m courses.

The US government has ordered 10m courses of the Pfizer drug at a price of $530 per course compares to the deal with Merck for as many as 5m courses of molnupiravir at a price of $700 per course.

Paul Schaper, Merck's head of global public policy, said the company will ship hundreds of thousands of treatment within several days and million courses of treatment within several weeks in the US.

Merck has said molnupiravir, which helps prevent the virus from replicating, should be effective against any variant, including the new Omicron variant.