The F.D.A. also required Biogen to conduct another clinical trial to determine if the drug provided any evidence of benefit. In the years it will take for that trial to be completed, Aduhelm will be available to patients.

Medicare’s coverage decision is considered pivotal to the future of Aduhelm, which is priced at $28,200 a year per patient. Roughly 80 percent of the patients eligible for the drug are old enough to receive coverage under Medicare.

Biogen initially priced the drug at $56,000 a year, but slashed the price in half last month after months of weak sales. Before Biogen’s price cut, Medicare’s actuarial division, acting without knowing what the coverage decision would be, imposed one of the biggest-ever increases in Medicare Part B premiums for 2022, partly driven by the possibility of Aduhelm coverage.

After Biogen’s price cut, advocacy groups for Alzheimer’s patients urged C.M.S. to lower the premium increase. On Monday, the secretary for health and human services, Xavier Becerra, said that he was instructing C.M.S. to re-examine the premium hike “given the dramatic price change of the Alzheimer’s drug, Aduhelm.”

Medicare officials are supposed to determine coverage based on whether a drug is a “reasonable and necessary.,” and in a briefing with reporters, officials said price wasn’t a factor in their decision.

Dr. Fleisher, acknowledging the unusual nature of the decision, said that “C.M.S. is using its authority provided by Congress to determine if the drug is considered reasonable and necessary, meaning that the benefits of improvement of cognition outweigh the harms in the Medicare population.”

Several advocacy groups for patients, including the Alzheimer’s Association, had pushed for approval of the drug and for Medicare coverage, and they expressed strong disappointment in the agency’s preliminary decision.