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Pfizer’s COVID-19 antiviral medication Paxlovid—the first such pill to be approved by the U.S. Food and Drug Administration (FDA) in May—has become the most common medicine used for treating adults who are at high-risk for experiencing a severe case of COVID-19.

Clinical trials have shown it to be effective: a February 2022 report found that it reduced the risk of progression to severe Covid-19 in most patients by 89%.

“It remains an important treatment option for people at high risk for progression to severe COVID-19, including those with prior immunity,” said Patrizia Cavazzoni, M.D., director for the FDA’s Center for Drug Evaluation and Research, in a May press release.

With COVID-19 cases increasing again in the U.S. and Paxlovid becoming more widely available, it’s useful to understand a bit more about the drug, its efficacy, and who can get it.

What is Paxlovid and how does it work?

Paxlovid is made up of two medications: nirmatrelvir, which inhibits the SARS-CoV-2 protein from replicating, and ritonavir, which is best known as an HIV/AIDS treatment that is now used to stop antivirals such as nirmatrelvir from breaking down too quickly so your body can keep fighting the infection. Patients prescribed Paxlovid take a total of 30 pills: three pills twice a day for five days.

How effective is Paxlovid for treating COVID-19?

Based on studies conducted so far, Paxlovid should be prescribed within five days of symptoms appearing for the best chance of success.

Studies have shown that Paxlovid has strong efficacy for treating COVID-19 for patients who are at-risk at developing a severe case. It is particularly effective among adults who are unvaccinated, had never had COVID-19, and were either 18 years of age and older and at risk of progressing to severe disease, or older than 60.

A clinical trial that evaluated COVID-19’s effect on high risk patients referred to by the FDA as the EPIC-HR trial, showed that Paxlovid reduced the number of COVID-19-related hospitalizations or deaths 28 days after contracting the virus by 86%. The study, completed between July and December 2021, looked at patients who were symptomatic and unvaccinated. Other studies also show that the drug has limited benefits for people who do not have risk factors for serious disease.

In March 2022, Pfizer announced the launch of a clinical trial studying the safety and success of Paxlovid in children and teenagers between 6 and 17 years old. The results of that trial have not yet been released.

In 2022, when prescriptions for Paxlovid were initially on the rise, the treatment was “remains highly effective and is mainly well tolerated,” with the different variants, Dr. Zenobia Brown, medical director and vice president of population health care management at Northwell Health, told TIME. “However, we may be facing a situation where a longer course is warranted for some patients, especially those who receive the drug very early in their disease course and/or are immunocompromised.”

But scientists are not all that sure of its efficacy as the virus continues to mutate. Research has shown that Paxlovid works against Omicron subvariant EG.5, which is the dominant strain in the U.S. It is still not clear if Paxlovid works against Omicron subvariant BA.2.86, a coronavirus variant first detected in late July that has more than 30 mutations. Doctors say that means the virus has the “potential to bypass immune defenses both from natural infection and prior vaccination,” according to Yale Medicine.

How can you get Paxlovid?

Paxlovid is currently authorized for use in people ages 12 and up, weighing at least 88 pounds, who test positive for COVID-19 and are at high risk of developing severe illness. This includes individuals who have underlying conditions such as cancer or diabetes or who are 50 or older.

If you are pregnant or breastfeeding, the FDA says you should talk to your doctor about using Paxlovid.

Aside from physicians, state-licensed pharmacists can also prescribe Paxlovid under the emergency use authorization issued by the FDA

What are the side effects of Paxlovid?

There are a few side effects to watch out for. One is dysgeusia, which negatively affects your sense of taste; it’s a side effect also listed for other drugs like antibiotics, chemotherapeutics, and antihistamines. Over 5% of the participants in Pfizer’s clinical trial experienced this side effect.

While each person’s experience can vary, many people who get this describe it as having a bitter or metallic taste in their mouths. “It tastes terrible—but not terrible enough to make most people stop therapy,” says Dr. Paul Sax, professor of medicine at Brigham and Women’s Hospital and Harvard Medical School.

Other common side effects include diarrhea, muscle aches, and high blood pressure. “These side effects are relatively minor, and the drug is well tolerated by most,” says Brown. “The larger issue with Paxlovid… is that the drug is metabolized by the same enzymes that metabolize many common medications. In some instances, those interactions can increase or decrease drug levels dangerously or reduce the amount of drug and efficacy of the Paxlovid.” For example, she points out that “patients who are on hormonal contraceptives are asked to use a backup method.”

Ritonavir can also cause liver problems, so patients with a history of liver problems or pre-existing liver diseases, liver enzyme abnormalities, or hepatitis, should talk to their doctor before taking the medicine. Paxlovid is also not recommended for patients with severe kidney problems, while those with moderate kidney issues may need a different dosage level.

Both active ingredients in Paxlovid may also interact with other medications—such as immunosuppressants for organ transplant recipients and blood thinners—so the FDA advises people to first talk to their doctor about any medications they are taking.

Can Paxlovid help with Long COVID?

There is some evidence that Paxlovid can help with long COVID.

An analysis of federal health data by researchers at Washington University School of Medicine in St. Louis and the Veterans Affairs St. Louis Health Care system in March found that Paxlovid decreased the probability of contracting long COVID by 26% over the course of six months.

“Our research reinforces such theories. It stands to reason that an antiviral drug—one that suppresses viral replication—may reduce the risk of long COVID,” said senior author Ziyad Al-Aly, MD, a Washington University clinical epidemiologist.

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