U.S. health insurers say they want more proof before paying for Biogen Inc.’s Aduhelm, stalling sales of the costly new Alzheimer’s therapy that the company hailed as a breakthrough for patients.

None of the 25 large insurers that responded to a Bloomberg News survey judged the $56,000-a-year drug “medically necessary,” a term used to describe treatments that are needed for specific ailments and meet medical standards. Most have deemed Aduhelm experimental, while some say they’re still evaluating it.

Insurers cited uncertainty about benefits and side effects for their denials. Still, Humana Inc., the nation’s second-largest Medicare insurer, is covering the drug for members who resemble those it helped in clinical trials.

Health insurers and government programs will play an outsize role as gatekeepers for Aduhelm, which the Food and Drug Administration approved despite objections from its scientific advisers and contradictory clinical trial results. Some patients are paying for the therapy out-of-pocket or seeking free supplies, while doctors who want to prescribe the drug must work hard to justify it.

For each person deemed eligible for Aduhelm, staffers can spend five hours trying to get insurers’ approval, said Paul Schulz, a doctor with the McGovern Medical School at UTHealth Houston and Memorial Hermann-Texas Medical Center. It’s rarely granted.

“It’s very difficult to spend that much time getting something approved on a case-by-case basis,” said Schulz, who’s treating about half a dozen people with Aduhelm. He’s received fees from Biogen for giving speeches.

Medicare, which covers 63 million elderly or disabled Americans, won’t set its final policy for Aduhelm until April, though the program is already raising premiums in anticipation of demand. Private insurers say they need more evidence that the treatment actually slows the rate at which Alzheimer’s patients deteriorate.

Aduhelm isn’t medically necessary for Alzheimer’s treatment “as a clinical benefit has not been established,” a representative for Health Care Service Corp. said in an email. The company has almost 17 million members in Blue Cross Blue Shield plans across five states, including Texas.

Biogen didn’t respond to requests for comment for this story.

Doctors, patients and families are desperate for relief from memory-robbing Alzheimer’s that affects some 6 million Americans, most of them elderly. Aduhelm removes amyloid plaques, protein deposits that are believed to play a role in the condition’s attack on the brain.

Questions about the drug’s effectiveness have persisted since it wobbled to the U.S. market, with evidence so controversial that three FDA advisers resigned after its June approval. Europe’s regulator has indicated it won’t recommend clearance.

While critics warned that the costly therapy threatened to overwhelm government health programs, sales have been just a trickle, disappointing investors in Biogen and its partner Eisai Co. Biogen’s stock has surfed the drug’s fortunes. The shares neared $415 in June when prospects for use still seemed bright; since then, they’ve dropped by more than a third.

Some patients’ prescriptions are covered. Kaiser Permanente, the giant California-based health maintenance organization, said it pays for the drug when a member’s physician determines it’s medically necessary.

Humana’s decision to cover patients who meet the trial criteria isn’t final, a spokesman said. The Medicare-focused insurer remains concerned about conflicting data on the drug and said the health program’s decision will influence its final policy. Neither company said how many members have received the drug.

Other insurers are more restrictive. In Biogen’s June news release announcing the drug launch, a Cigna Corp. executive was quoted saying that the company was “working to ensure that the patients who will benefit most from this new treatment have a clear path to access it.”

Since then, though, Cigna called the drug “unproven” and won’t cover it, according to its policy. In an emailed statement, the company cited “emerging concerns regarding its safety and efficacy.”

At First Choice Neurology, a large chain of clinics in south Florida, about 80 patients are receiving Aduhelm, according to Jeffrey Gelblum, a doctor at the practice. Some commercial insurers are approving claims for the drug, he said, without providing names.

Many patients who haven’t gotten a green light get the drug for free from Biogen at least temporarily while the practice appeals to their insurers, he said, which can take months. Gelblum said that’s problematic for a drug most likely to work early in the disease.

“I don’t want to waste time,” he said in an interview. “You’ve got a two-year window of intervention here, and if a patient comes in saying, ‘Look I’ve had this problem for a year,’ well, now I know I’m already a year into the problem.”

Determining whether patients qualify for treatment can also be a barrier. One way of looking for telltale amyloid plaques is a PET scan of the brain. The procedure costs about $5,000, and insurers won’t pay.

Some of Marwan Sabbagh’s patients at the Barrow Neurological Institute in Phoenix have paid for the scan on their own. Another option is a spinal tap. Biogen may even cover the cost of analyzing the test. Yet many people fear the procedure. The neurologist has offered some entry to a clinical trial where the scan and treatment are paid for.

“There are many bottlenecks,” said Sabbagh, who’s been paid to advise Biogen. “It’s not exclusive to our healthcare system.”

Large insurers that haven’t publicly disclosed their positions on Aduhelm and didn’t respond to Bloomberg’s questions include Anthem Inc., CVS Health Corp. and Centene Corp. UnitedHealth Group Inc. referred to comments from July, when the company said it was still evaluating the drug and awaiting guidance from Medicare.

While the government health program is still deciding whether to cover Aduhelm, millions of patients will soon pay either way: Medicare premiums will rise by about $11 per person per month in 2022 to prepare for the potential costs of the drug and similar therapies that may follow.

Other insurers said they would cover Aduhelm in their plans that administer Medicaid, the state-federal health program for low-income people, if required. Molina Healthcare Inc., which has 4.8 million members primarily in Medicaid plans, said Aduhelm would only be covered as called for by states.

The drug isn’t covered in the company’s offerings through the Affordable Care Act marketplace, where individuals and families can buy government-subsidized health plans. Molina hasn’t received any requests for coverage yet, a spokeswoman said.

State Medicaid programs are required to cover the drug because Biogen participates in the Medicaid drug rebate program, a spokesperson for the U.S. agency said in an email.

Most insurers that responded to Bloomberg’s questions said they would continue to evaluate new evidence and take Medicare’s decision into account when it comes. As a condition of the drug’s approval, Biogen must do another study to confirm Aduhelm’s benefits. Adding to the turmoil, Biogen said this month that 64-year-old research chief Alfred Sandrock, a key figure behind Aduhelm, will retire at the end of the year.

Meanwhile doctors and patients who want the drug are seeking it through clinical trials or Biogen’s access program. Jonathan Liss, founder of Columbus Memory Center in Georgia, said two of his patients are getting the drug through the company program, and he’s personally covering the costs for a third.

Liss, who has received money from Biogen to serve on advisory boards, said he’s wary of starting patients on the drug who might have to stop if Medicare decides not to cover it.

“What I’ve said to a number of patients is ‘Look, I can get you the drug,'” he said, “‘but I don’t know if it’s going to get paid for.'”