Because the pill works faster than other antidepressants and is taken for only two weeks, it may encourage more treatment of the debilitating condition.
The F.D.A. gave full approval to the drug, but added a black-box warning about safety risks. Medicare said it would cover most of the high cost.
The drug, Leqembi, may modestly slow cognitive decline in early stages of the disease but carries some safety risks. Still, data suggests it is more promising than the small number of other available treatments.
The drug, lecanemab, made by Eisai and Biogen, also carried risks of brain swelling and bleeding and should be studied further, a report of the findings said.
Officials cited data showing the new Alzheimer’s drug has serious safety risks and may not help patients.
The Alzheimer’s Association has pushed relentlessly to get broad access to Aduhelm, despite safety risks and uncertain evidence that it helps patients.
If the preliminary decision is finalized this spring, it would sharply limit the number of patients who use the expensive drug.
Medicare officials are trying to determine whether to cover Aduhelm, which the F.D.A. approved despite unclear evidence that it helps patients.
A group of Alzheimer’s experts and health advocates called on the F.D.A. to withdraw its approval of the drug, the latest of several setbacks for the treatment.
Newly published safety data shows that 41 percent of patients in key clinical trials of the Alzheimer’s drug experienced brain bleeding or swelling, though many cases were asymptomatic.