Under fire for approving a questionable drug for all Alzheimer’s patients, the Food and Drug Administration today greatly narrowed its previous recommendation and is now suggesting that only those with mild memory or thinking problems should receive it.

The reversal, highly unusual for a drug that has been available for only a few weeks, is likely to reduce the approximate number of Americans who are eligible for the treatment to 1.5 million from 6 million.

The approval of Aduhelm early last month was one of the most contentious FDA decisions in years. Groups that represent Alzheimer’s patients had intensely lobbied the agency to sign off on the first new drug to treat the disease in 18 years — and the first ever designed to attack its biological underpinnings.

But many scientists, as well as the FDA’s independent advisory committee, said there was not convincing evidence that the drug worked.

In addition, the agency’s recommendation that Aduhelm be available to all Alzheimer’s patients, not just those showing early symptoms, stirred up even more concern among medical experts, including those who had supported the drug’s approval.

After the approval, three members of the advisory committee resigned in protest. One, Dr. Aaron Kesselheim, described it as “the worst approval decision” that he could remember.

The drug’s maker, Biogen, said last month that it would charge $56,000 annually for the drug. Associated costs — such as those for diagnostics and safety monitoring, since the drug’s side effects include brain swelling and bleeding — could add tens of thousands of dollars to each patient’s annual bill.

Analysts expected that the drug’s widespread use would strain Medicare’s budget. By one estimate, it could leave taxpayers on the hook for $29 billion in new spending, more than the annual budget of the NASA.

The new guidance does not prevent doctors from prescribing Aduhelm to patients with moderate or severe Alzheimer’s. But the about-face sends a strong message to doctors and insurers about who should receive the drug.

It also substantially increases the odds that Medicare and private insurers will restrict coverage of the drug, which is given as a monthly intravenous infusion. That would mean that patients with moderate or severe Alzheimer’s would have to pay the five-figure annual costs out of their own pockets, which experts regard as unlikely to happen frequently.

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