FDA approves new flu pill that can be taken after onset of symptoms

The FDA has approved a new flu medication in the form of a single-dose pill.

The medication, called XOFLUZA, was designed to make the flu shorter if taken within 48 hours of onset and can be prescribed to patients 12 and older once they've been diagnosed with the illness, according to the FDA.

The new medication is designed to significantly reduced the duration of flu symptoms by more than one day, when compared to a placebo in clinical trials, the FDA said.

Researchers believe the medication will prevent patients from spreading the flu, which could help reign in contagion rates following the 2017-2018 flu season, which saw 900,000 hospitalizations and 80,000 deaths -- the deadliest season in four decades, according to the FDA. The medication was approved under a priority review.

The medication is the first in a new class of antiviral medications that blocks an enzyme within the flu virus, which stops viral replication early in the influenza cycle, the FDA announced.

XOFLUZA is similar to Oseltamivir, also known under the brand name Tamiflu, but "decreases viral shedding faster" and is associated with less side effects, Dr. Todd Ellerin, the infectious disease specialist at South Shore Hospital in Massachusetts, told ABC News.

"Like Tamiflu, it seems to work better the quicker it gets into the patient," Ellerin said.

In clinical trials, XOFLUZA demonstrated efficacy against a wide range of influenza, including Tamiflu-resistant strains, and has also proved effective against some strains of Asian bird flu, such as H5N1, according to the FDA.

The medication is the "first new flu medicine" of its kind -- with a novel proposed mechanism -- in nearly 20 years, said Dr. Sandra Horning, chief medical officer and head of global product development at Genentech, the company selling the pill in the U.S., in a statement to ABC News.

"If patients see their doctors within 48 hours of symptom onset, one dose of XOFLUZA can significantly reduce the duration of flu symptoms," Horning said.

Since it was tested in healthy, adult patients, it is unclear how effective it will be in the most severe cases or for the most vulnerable patients, Ellerin said.

The pill will be available across the U.S. in the coming weeks, according to the FDA.