Sarepta Therapeutics srpt won a surprising victory last month when regulators approved its experimental drug for a muscle-wasting rare disease following a concerted lobbying campaign by the biotech’s allies and patient advocates. But the controversial therapy is already running into headwinds with skeptical health insurers.

Insurance titan Anthem antm , which has more than 38 million health plan holders, has decided not to offer coverage for the new treatment, eteplirsen (sold under the brand name Exondys 51), the company announced in a medical policy update. That could prove to be a major barrier to patients and their families since the drug has a topline cost of $300,000.

Exondys 51 was approved by the Food and Drug Administration (FDA) in September as the first treatment for Duchenne muscular dystrophy (DMD) in the U.S. It’s approved to treat a certain form of the muscular disease which affects about 13% of the DMD population, or somewhere between 1,300 and 1,600 people in America. The genetic disorder overwhelmingly affects males and saps their movement abilities, often killing patients by the time they turn 30.

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But the FDA turned heads with its approval because the agency essentially overruled the recommendation of its own independent advisers to deny Exondys 51 regulatory clearance. That initial vote of no confidence, based on critiques that Sarepta’s trials involved far too few participants and didn’t demonstrate a significant benefit to patients, was also split in a close 7-6 vote, underscoring the intense intra-agency disagreements over how to approach the therapy.

Skeptics of the treatment, including the FDA’s Dr. Ellis Unger, warned that approving the drug would ultimately lower the FDA’s regulatory standards since the agency is supposed to approve medicines that are empirically proven to be both safe and effective—the latter metric, Unger argued, was not cleared by Exondys 51. But Dr. Janet Woodcock of the FDA (Unger’s superior) pushed for approval, swayed by Sarepta’s and patient advocates’ argument that it would be irresponsible not to approve a treatment for a disease that has no treatments, even if its effects may appear modest.

The FDA ultimately decided to approve the drug conditionally. Sarepta will be required to conduct a follow-up study proving the therapy’s clinical benefit, and it could potentially be pulled from the market if the firm fails to do that.

But Anthem’s decision not to cover the drug’s cost underscores the potential problems with the FDA’s decision. By approving a drug without confirming its clinical benefit, there’s a chance that Duchenne patients’ families will be on the hook for subsidizing an expensive treatment that has yet to be definitively proven as effective. And that be particularly burdensome if Sarepta’s follow-up trials don’t confirm its efficacy.

Duchenne advocates argue that any option is better than none when it comes to such a devastating illness. But as Anthem’s announcement shows, that’s not necessarily the same rationale being used by all parties in the health sector—particularly insurers who want to see proof of improved health outcomes before doling out coverage.