(NEXSTAR) — The White House confirmed on Thursday that President Joe Biden has tested positive for COVID-19. While he is said to be experiencing mild symptoms, press secretary Karine Jean-Pierre said that Biden is taking Paxlovid.

But what is Paxlovid?

Paxlovid is an antiviral therapy produced by Pfizer and given to patients with COVID-19. The Food and Drug Administration granted emergency authorization for the pill in December, making it the first approved for use against COVID-19 in the U.S. It’s intended to reduce the severity of COVID symptoms.

The pill is available by prescription, and the FDA recently authorized state-licensed pharmacists to prescribe it to eligible patients. Patrizia Cavazzoni, director of the FDA’s Center for Drug Evaluation and Research, said the move “could expand access to timely treatment for some patients who are eligible.” Previously, only doctors could prescribe the pill.

Paxlovid must be taken within five days after COVID symptoms begin, according to the FDA.

Currently, the antiviral pill is only authorized to treat patients 12 years old and older who weigh at least 88 pounds after they test positive for COVID. Paxlovid can be used in patients with mild to moderate COVID that are at high risk of progressing to severe COVID, which could lead to hospitalization or death.

This includes older people – Biden is 79 – and those with health issues like heart disease, obesity, cancer, or diabetes.

Paxlovid is given as three tablets: two nirmatrelvir tablets and one ritonavir tablet. The nirmatrelvir inhibits a SARS-CoV-2 protein, which stops the virus from replicating, and the ritonavir, which slows the breakdown of the nirmatrelvir, the FDA explains. The three tablets are taken together twice a day for five days.

Paxlovid isn’t approved to prevent a person from getting COVID, or in place of getting a vaccine.

The U.S.’s top infectious disease expert, Dr. Anthony Fauci, took Paxlovid last month after testing positive for COVID. He later said he had a recurrence of symptoms after taking the antiviral pill.

While speaking at a global health forum, Fauci said he tested negative for COVID for three consecutive days after finishing his five-day Paxlovid series. On the fourth day, though, Fauci said he tested positive and began having worsening symptoms.

He referred to it as “Paxlovid rebound,” which the Centers for Disease Control and Prevention acknowledged may happen. The agency issued an advisory in May that patients may again have symptoms and a positive COVID test within two to eight days of finishing their Paxlovid series.

Fauci ultimately did a second round of Paxlovid, which the CDC has not determined to be necessary if patients experience a rebound of symptoms. Instead, experiencing symptoms after taking the pill “may be part of the natural history” of the virus.

Biden is now the second president to test positive for COVID. Former President Trump contracted the virus in October 2020 and was hospitalized for three days at Walter Reed National Military Medical Center.

Melanie Vásquez Russell and Michael Bartiromo contributed to this report.