A pill that helps control flu symptoms after one dose is getting fast consideration from the Food and Drug Administration, the company that makes it said Tuesday.

Genentech said the FDA had granted priority review to baloxavir marboxil and would decide whether to approve it by the end of this year.

If approved, baloxavir would be the first new flu drug to hit the U.S. market in years and the first with a new mechanism of action in 20 years.

“Baloxavir marboxil has been shown in clinical trials to decrease the duration of symptoms with one dose, and demonstrated a significant reduction in viral shedding in just one day,” said Dr. Sandra Horning, chief medical officer at drug giant Roche, which owns Genentech.

Influenza viruses developed resistance long ago to the first two antiviral flu drugs, amantadine and rimantadine.

Newer antiviral drugs on the market include Tamiflu, a pill; Relenza, which can be inhaled; and an injectable drug called peramivir.

All three are in a class called neuraminidase inhibitors. They help stop the virus from spreading inside the body and work best when given within a day or so of symptoms starting.

Even if the antivirals don’t cure flu the way an antibiotic can cure strep throat, they’re still useful, the CDC says. They reduce how long people are sick with flu and make it less likely to kill them. Tamiflu can also be used to prevent infection in really vulnerable groups, such as young children and nursing home residents.

Baloxavir, already approved for use in Japan, works in a different way. It interferes with a protein inside the flu virus called cap-dependent endonuclease protein.

It helps stop virus replication, also, but earlier on in the process. "It inhibits the virus much earlier in the influenza life cycle," said Genentech vice president Mark Eisner.

"It's going to be more convenient than the neuraminidase inhibitors because it works after a single dose," Eisner told NBC News.

A 2016-2017 study in 1,436 people in the U.S. and Japan showed the one-dose pill cut the time people were sick to 2.5 days from about 3.3 days. It cut how long people had a fever from an average of 42 hours to just one day and reduced what’s called viral shedding from four days to just one day.

Reducing viral shedding should, in theory, limit the spread of the virus from person to person, although the company has not shown that.

It did not cause many side-effects.

The company has continued testing it in more people, including people at high risk of flu complications, and is analyzing the results now, Eisner said.

Tamiflu is now recommended for children and people who have a high risk of flu, but patients must take multiple doses over several days. Relenza is less commonly used, but it and peramivir are more useful for hospitalized patients who may be unable to take pills.

Doctors like having a variety of drugs on hand for any disease, but especially influenza, which mutates quickly and which can develop resistance to drugs. In 2009, for instance, the H1N1 swine flu strain developed resistance to Tamiflu.

Baloxavir, developed with support from the National Institutes of Health, is sold in Japan by Shionogi & Co. under the name Xofluza. Genentech has not said what the U.S. name would be if the FDA approves it.

“The FDA is expected to make a decision on approval by December 24, 2018,” the company said. Company representatives said it was too soon to talk about a name for the U.S. product or what it might cost.