WASHINGTON — The Food and Drug Administration has approved the first drug to treat Duchenne muscular dystrophy, a rare and lethal muscle weakening disorder that affects about 15,000 Americans.

The FDA’s approval of the drug, Exondys 51, also known generically as eteplirsen, came over the objections of its own advisory committee, which voted not to approve the medication earlier this year. Patients and their families had lobbied hard for the drug, made by Sarepta Therapeutics of Cambridge, Mass., noting that people with the disease have few treatment options.

Exondys 51 could sell for as much as $350,000 a year, according to Michelle Gilson, an analyst for Oppenheimer & Co., an investment bank.

Exondys 51 doesn’t cure Duchenne muscular dystrophy and will only help a minority of patients. It is designed for the 13 percent of patients with a particular genetic mutation that prevents them from making dystrophin, a key protein that keeps muscles intact. Without that protein, muscles weaken so that children are unable to walk and must use wheelchairs by the time they’re teens. Eventually, the disease can fatally weaken the heart and muscles needed to breathe. Patients often die in their 20s or 30s.

The decision speeds up the approval process for Exondys 51, allowing it onto the market based on preliminary data that suggests the drug will strengthen children’s muscles.

In clinical trials, some patients treated with Exondys 51 had more dystrophin in their skeletal muscles, which people use to move their arms and legs. The FDA will require Sarepta to launch another clinical trial to show whether it improves patients’ symptoms.

Duchenne muscular dystrophy occurs in about one out of every 3,600 baby boys worldwide.