May 9, 2022 6:10 PM EDT

Prescriptions for Pfizer’s COVID-19 antiviral medication Paxlovid—the first such pill to be authorized by the U.S. Food and Drug Administration (FDA)—have increased tenfold in the past two months. Between late February and April 22, the number of patients using Paxlovid jumped from 8,000 to 80,000, according to Pfizer CEO Albert Bourla; the drug is now available at some 33,000 locations, such as pharmacies and outpatient clinics, across the country.

On April 26, the White House announced it would step up its efforts to encourage more doctors to prescribe the antiviral medication. As part of this, the Biden Administration announced the drug will be distributed directly to pharmacies (in addition to already being available at federal test-to-treat sites). In doing so, it expects the number of sites across the country giving out Paxlovid will eventually increase to 40,000. “The bottom line is that we want to make this therapeutic available to all Americans,” Dr. Ashish Jha, the White House COVID-19 response coordinator, told CNN.

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. Taking the pills within this window has been shown to lower the risk of severe illness or hospitalization by 89% among people most vulnerable to the worst effects of COVID-19. This data is based on a Pfizer clinical trial conducted during the second half of 2021 among unvaccinated adults. The company is now in the middle of a clinical trial studying the safety and success of Paxlovid in children and teenagers between 6 and 17 years old.

And while the clinical trials were done before Omicron took off, Pfizer says Paxlovid does work against the variant. Lab results detailed in a March study published in the New England Journal of Medicine also found that the main ingredients in Paxlovid were effective against the Omicron subvariant BA.2.

Paxlovid “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.”

Dr. Paul Sax, professor of medicine at Brigham and Women’s Hospital and Harvard Medical School, adds that “the most important new information is that some people will experience a relapse of COVID symptoms a few days after stopping treatment. People who have this relapse should consider themselves potentially contagious to others, especially if they test positive again on an antigen test.”

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 65 or older.

Some experts, however, are calling for doctors to start prescribing the pills even before people who test positive for COVID-19 start experiencing symptoms. “Because initially we had a shortage of drug supply, people got the message we should be very reserved and only use it for the very higher source people when they get sick,” FDA Commissioner Robert M. Califf told SiriusXM’s Doctor Radio Reports host Dr. Marc Siegel on May 6. “But in fact, what we want to do is to use it before they get sick—and people who are significantly at risk, older age or multiple comorbidities, for sure we want to treat those people early.”

“A problem that we’re seeing is people come in or get diagnosed, they’re feeling okay, but they’re high risk,” he continued. “And if you wait until they get sick, it’s too late. You want to prevent them from getting sick.”

Meanwhile, preliminary data suggests that Paxlovid may also protect people who aren’t considered high-risk, potentially bolstering the argument for the drug to be given to the wider population. Analysis based on an ongoing Pfizer trial found participants with COVID-19 who were given Paxlovid were 70% less likely to be hospitalized than those who weren’t given the drug.

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 Sax.

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, and why many providers are reluctant to use it, 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 isn’t much data on this yet. So far, two patients with Long COVID—one of whom was a researcher who tested it on herself—saw their symptoms lessen or go away after taking Paxlovid. In one case, a person reported that their acute symptoms disappeared after 48 hours, but their intense fatigue continued.

Anecdotally, “it would appear that some of the factors that impact Long COVID and Paxlovid are the timing of the delivery as well as the likelihood of developing Long COVID based on previous vaccination and booster,” Brown explains. “In some cases, taking Paxlovid too soon after symptoms onset might not be protective. We are seeing a handful of rebound cases that could theoretically become Long COVID, since the inflammation in the end organs occurs in these rebound cases despite the early use of the drug. On the other hand, some studies would suggest a Long COVID benefit.” Much more research is needed.

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