Paxlovid is a drug that can be used immediately after signs and symptoms appear in patients at high risk for severe disease.
It reduced hospitalization and death rates by 89% in vulnerable adults through clinical trials.
The tablet, which partly uses an existing HIV medication that was developed by Pfizer in the USA, has been ordered by 2.75 million people in the UK.
After molnupiravir in November, Paxlovid, the UK's second approved antiviral drug for Covid, was granted the green light by regulators.
Sajid Javid, the health secretary, said that new antiviral drugs and booster vaccines combined with increased testing means the UK is "in the strongest position" to combat Omicron's threat in 2022.
Paxlovid is known as a protease inhibitor. It blocks an enzyme that the virus requires to multiply. It stays longer in the body when taken with a low dose ritonavir.
It is approved in the UK for patients over 18 who have a mild or moderate Covid infection and are at high risk of having their condition worsen.
Clinical trials in 1 219 patients with the virus showed that 0.8% of those who received Paxlovid were admitted to hospital after they had recovered. This compares with 7% for patients who received a placebo or dummy pills.
These results were released in a press release, but have not been peer reviewed by any other scientists.
Dr June Raine is the chief executive officer of the UK Medicines and Healthcare products Regulatory Agency. She stated: "We now possess an additional antiviral medicine to treat Covid, which can be taken orally rather than intravenously."
"This allows it to be administered outside of a hospital setting before Covid has advanced to a severe stage."
Paxlovid's effectiveness is greatest when it is taken early in the infection. The regulator recommended that the drug be administered within five days of onset of symptoms.
Doctors advise that it is crucial to quickly provide antiviral medication to those who have tested positive if they are to have an impact on the death and hospitalization rates.
Paxlovid is likely to be prescribed by GPs. It can be picked up from a local pharmacy and sent directly to those infected. In an effort to speed up treatment, some of the most vulnerable to Covid have been given PCR tests.
Molnupiravir is another antiviral medication that is being offered to vulnerable adults in the UK. This includes transplant recipients and cancer patients. As part of an extensive study by Oxford University, it is being tested on more than 10,000 older people.
The UK ordered 2.3 million more molnupiravir from US drug company Merck Sharp & Dohme earlier this month. The French health authorities also cancelled their entire order for molnupiravir after receiving disappointing results from clinical trials.
Sotrovimab is a third treatment that has been approved for this disease. It can be administered infusionally in hospitals and clinics to outpatients.
