The FDA approved a treatment costing $3.5 million, making it the world's most expensive medicine.
Hemgenix effectively treated several patients with the blood condition Hemophilia B in trials.
An independent study said a fair price for the drug would be about $2.9 million.
US regulators have approved a hemophilia drug that will cost $3.5 million per patient, making it the world's most expensive medicine.
The Food and Drug Administration (FDA) announced on Tuesday that it had approved Hemgenix, the first gene therapy to treat adults with hemophilia B, a genetic bleeding disorder resulting from missing or insufficient levels of blood-clotting Factor IX.
The FDA said the condition affected one in 40,000 people, mostly men. It accounts for about 15% of all hemophilia cases.
In a study Hemgenix, distributed by CSL Behring, cut the number of bleeding events expected over a year by 54%. It also removed the need for 94% of patients to receive infusions of Factor IX, saving them considerable time and money.
"Gene therapy for hemophilia has been on the horizon for more than two decades," said Peter Marks, director of the FDA's Center for Biologics Evaluation and Research.
"Today's approval provides a new treatment option for patients with hemophilia B and represents important progress in the development of innovative therapies for those experiencing a high burden of disease associated with this form of hemophilia."
However, the drug will have a list price of $3.5 million per dose, Managed Healthcare Executive reported, making it the world's most expensive medicine by some distance.
A spokesperson for CSL told the publication: "We are confident this price point will generate significant cost savings for the overall healthcare system and significantly lower the economic burden of hemophilia B by reducing annual bleed rates, reducing or eliminating prophylactic therapy and generating elevated FIX (factor 9) levels that last for years."
The price is higher that the figure of about $2.9 million recommended in an independent review by the Institute for Clinical and Economic Review.
Brad Loncar, a biotechnology investor and chief executive of Loncar Investments, told Bloomberg he thought the treatment could be successful because existing drugs were also very costly and that hemophilia patients "constantly live in fear of bleeds."
The list price for Hemgenix overtakes Bluebird Bio's Zynteglo, which treats the blood disorder beta thalassemia, that was priced at $2.8 million earlier this year.