The first new flu drug in 20 years won approval Wednesday from the Food and Drug Administration.

The new antiviral, generically known as baloxavir marboxil but sold under the brand name Xofluza, is a single dose treatment. It is for use only in those aged 12 or more, the F.D.A. said, and should be taken only in the first two days after symptoms like fever, aches and sniffles appear.

The pill will cost $150, according to a spokeswoman for Genentech, which will sell Xofluza in this country. Genentech will offer coupons that lower the price to $30 for patients with health insurance and to about $90 for the uninsured.

Flu season has already begun, and last year’s was one of the most lethal in decades; about 80,000 Americans died of flu or its consequences, the Centers for Disease Control and Prevention said earlier this month .

The new drug is no miracle cure and it is not approved for young children, who are one of the most vulnerable groups, so the C.D.C. still strongly recommends that every American over the age of six months get a flu shot as soon as possible.