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The Food and Drug Administration approved a controversial muscular dystrophy drug Monday, ignoring the advice of its advisers and delighting families and advocates who had campaigned hard for its approval.

The company that makes the drug, called eteplirsen, says it will charge $300,000 a year for treatment.

Hunter Gandee, 14, and his brother Braden Gandee, 7, who has muscular dystrophy Jeremy Wadsworth / The Toledo Blade via AP

The drug is designed for children with a genetic mutation that causes some cases of Duchenne muscular dystrophy, the most common type of muscular dystrophy — a degenerative disease that causes muscles to break down because cells produce faulty versions of a protein called dystrophin, or none at all.

Studies of the drug cannot show whether it actually helped patients, but the FDA says the company can study it more and report back.

The disease affects more boys than girls and symptoms usually show up when kids are between 3 and 5. Right now there's no treatment, and patients almost always die young.

FDA’s panel of expert advisers recommended against the drug's approval after an emotional, packed meeting in which children with muscular dystrophy pleaded for the drug’s approval and doctors debated whether the drug helped everyone.

"I can hardly breathe," Jenn McNary, mother of two boys with muscular dystrophy in Saxtons River, Vermont, said by email.

"This is what success feels like. I can't wait to hug the boys."

And several FDA scientists fought hard to prevent its approval.

"FDA should never mislead patients by granting even accelerated approval to products that are not shown to offer the prospect of meaningful benefit to patients."

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"By allowing the marketing of an ineffective drug, essentially a scientifically elegant placebo, thousands of patients and their families would be given false hope in exchange for hardship and risk," Dr. Ellis Unger, acting director of FDA's Office of New Drugs, wrote in a memo released online.

"Eteplirsen’s risks are certain, whereas its efficacy is not," he added, noting that the drug will be delivered by a semi-permanent cather that could make kids vulnerable to infections.

"FDA should never mislead patients by granting even accelerated approval to products that are not shown to offer the prospect of meaningful benefit to patients under the appropriate regulatory and scientific standard," said Dr. Luciana Borio, acting deputy chief scientist at the FDA.

The agency gave itself extra time to decide. But expectations about approval soared last week when one key FDA critic of the drug, Ronald Farkas, left the agency last week.

Farkas has been the target of social media attacks by advocates for the drug who have accused him of holding up approval.

It's an unusual decision and the FDA says it's not clear whether the drug has actually helped any of the patients who have tried it.

“Patients with a particular type of Duchenne muscular dystrophy will now have access to an approved treatment for this rare and devastating disease."

"A clinical benefit of Exondys 51, including improved motor function, has not been established. In making this decision, the FDA considered the potential risks associated with the drug, the life-threatening and debilitating nature of the disease for these children and the lack of available therapy," the agency said.

Related: Does This Muscular Dystrophy Drug Work?

The FDA says it’s hoping for more information about how and whether eteplirsen works. It’s designed to help only about 13 percent of Duchenne muscular dystrophy patients.

“Patients with a particular type of Duchenne muscular dystrophy will now have access to an approved treatment for this rare and devastating disease,” said Dr. Janet Woodcock, director of the FDA’s Center for Drug Evaluation and Research.

“In rare diseases, new drug development is especially challenging due to the small numbers of people affected by each disease and the lack of medical understanding of many disorders,” Woodcock added in a statement.

We’re so excited that #FDA has approved #eteplirsen. Thx to all that helped us fight for this to happen. https://t.co/jT7EMHuYEp pic.twitter.com/bKfeBYIWGS — Jett Foundation (@JettFoundation) September 20, 2016

“Accelerated approval makes this drug available to patients based on initial data, but we eagerly await learning more about the efficacy of this drug through a confirmatory clinical trial that the company must conduct after approval.”

FDA Commissioner Dr. Robert Califf backed Woodcock over the objections of others at the FDA.

"I do not find that she deviated from her responsibilities as Center Director, nor do I find that she succumbed to pressure from the patient community, the public, the press or others," he wrote in the FDA memo, which detailed the arguments for and against approval.

"This is what success feels like."

Related: FDA Takes More Time to Consider Muscular Dystrophy Drug

"Exondys 51 was approved under the accelerated approval pathway, which provides for the approval of drugs that treat serious or life-threatening diseases and generally provide a meaningful advantage over existing treatments," the FDA said.

The drug's maker, Sarepta Therapeutics, will be required to gather more information on patients who try it.