When Exondys 51 was approved to treat Duchenne muscular dystrophy, a deadly disease that puts kids in wheelchairs by the time they are teenagers, there was no proof the drug actually slowed the disease. That was seven years ago. The company still hasn't provided conclusive data to this day.

The drug's maker, Sarepta Therapeutics Inc., has so far reaped more than $2.5 billion in sales from Exondys 51 and two related drugs. All three were cleared via a US regulatory shortcut called accelerated approval that allows companies to market medications before completing definitive trials in cases where patients have few or no treatment options.

In the case of Exondys, Sarepta's confirmatory study didn't even begin until four years after it was approved and won't be completed until 2024, eight years after it started selling the drug.