The Food and Drug Administration has approved the first drug to treat patients with spinal muscular atrophy, a savage disease that, in its most severe form, kills infants before they turn 2.

“This is a miracle — seriously,” Dr. Mary K. Schroth, a lung specialist in Madison, Wis., who treats children who have the disease, said of the approval, which was made last week. “This is a life-changing event, and this will change the course of this disease.” Dr. Schroth has previously worked as a paid consultant to Biogen, which is selling the drug.

The drug, called Spinraza, will not come cheap — and, by some estimates, will be among the most expensive drugs in the world.

Biogen, which is licensing Spinraza from Ionis Pharmaceuticals, said this week that one dose will have a list price of $125,000. That means the drug will cost $625,000 to $750,000 to cover the five or six doses needed in the first year, and about $375,000 annually after that, to cover the necessary three doses a year. Patients will presumably take Spinraza for the rest of their lives.