Medicine pill is seen with Merck logo displayed on a screen in the background in this illustration photo taken in Poland on October 4, 2021.NurPhoto | NurPhoto | Getty ImagesLONDON — An antiviral pill found to be effective at treating Covid-19 has been approved by Britain’s medicines regulator Thursday, in a potentially game-changing way of treating the virus.Britain’s medicines regulator said it had approved the world’s first antiviral oral pill, known as molnupiravir, having found it to be “safe and effective at reducing the risk of hospitalisation and death in people with mild to moderate Covid-19 who are at increased risk of developing severe disease.” Britain is the first country to approve the drug.Developed jointly by Ridgeback Biotherapeutics and Merck, the pill works by interfering with the virus’ replication. This prevents it from multiplying, keeping virus levels low in the body and therefore reducing the severity of the disease, the U.K.’s Medicines and Healthcare products Regulatory Agency said. It can also be administered outside of a hospital.The MHRA said the decision “follows a rigorous review of its safety, quality and effectiveness” and made it the “first oral antiviral for the treatment of Covid-19 to be approved.”Based on the clinical trial data, the oral pill is most effective when taken during the early stages of infection. As such, the British regulator recommended its use as soon as possible following a positive Covid test and within five days of the onset of symptoms.The antiviral pill has been authorized for use in people who have mild to moderate Covid and at least one risk factor for developing severe illness, such as obesity, diabetes mellitus, or heart disease and anyone over the age of 60.’Game-changer’The U.K.’s Health Secretary Sajid Javid said it was a “historic day for our country as the U.K. is now the first country in the world to approve an antiviral that can be taken at home for COVID-19.””This will be a gamechanger for the most vulnerable and the immunosuppressed, who will soon be able to receive the ground-breaking treatment,” he said in a statement.The MHRA’s Chief Executive June Raine said that the pill will greatly facilitate the treatment of Covid, a virus that has led to the hospitalization of millions of people around the world, and immense stress on health services and health care staff.Over 5 million people have now died of Covid-19 globally.”Following a rigorous review of the data by our expert scientists and clinicians, we are satisfied that Lagevrio (molnupiravir) is safe and effective for those at risk of developing severe Covid-19 disease and have granted its approval,” she said.”Lagevrio is another therapeutic to add to our armoury against Covid-19. It is also the world’s first approved antiviral for this disease that can be taken by mouth rather than administered intravenously. This is important, because it means it can be administered outside of a hospital setting, before Covid-19 has progressed to a severe stage,” she said.U.S. approval?Merck and Ridgeback Biotherapeutics said that internal data indicated that the risk of hospitalization or death from Covid was cut in half by molnupiravir. Patients take four pills twice a day for five days.Merck shares were 3% higher in the pre-market Thursday.- CNBC’s Rich Mendez contributed reporting to this story.