WASHINGTON — A new $56,000-a-year Alzheimer’s drug would raise Medicare premiums broadly, and some patients who are prescribed the medication could face copayments of about $11,500 annually, according to a recently published research report.

The drug, called Aduhelm, was approved by the Food and Drug Administration last week and quickly sparked controversy over its price and questionable benefits. An FDA adviser called the decision “probably the worst drug approval decision in recent U.S. history,” in a letter he submitted when resigning over the decision.

The new analysis by the nonpartisan Kaiser Family Foundation estimated that if just 500,000 Medicare recipients are prescribed Aduhelm, it would cost the program nearly $29 billion a year, far more than any other medication.

“At this price, the cost of this one drug alone could top all others covered by Medicare, if it is used widely,” said Tricia Neuman, coauthor of the report.

Separately, Dr. Aaron Kesselheim of Harvard University became the third member of an FDA advisory panel that opposed the drug to step down over the decision. Last November the 11-member group voted nearly unanimously against recommending approval for the medication, citing flaws in company studies. The FDA is not required to follow such recommendations.

In his resignation letter, Kesselheim said that the FDA’s recent drug approval decisions would undermine public trust, medical innovation “and the affordability of the health care system.” Last week, two expert neurologists also quit the panel.

Aduhelm is the first Alzheimer’s medication in nearly 20 years. It doesn’t cure the life-sapping neurological condition, but the FDA determined that its ability to reduce clumps of plaque in the brain is likely to slow dementia. Many experts say that benefit has not been clearly shown.

The drug’s approval came as congressional Democrats are trying to build consensus around legislation that would empower Medicare to negotiate prescription drug prices.

The chairman of the Senate Finance Committee, Democratic Sen. Ron Wyden of Oregon, said Thursday the list price for Aduhelm was “unconscionable.”

Medicare has not made a formal determination on covering the Alzheimer’s drug, but cost traditionally does not enter into such considerations. Drugmaker Biogen has said it priced Aduhelm responsibly and has committed to no price increases for four years.

The Kaiser analysis also found that domino effects would include higher “Part B” premiums for Medicare’s outpatient coverage and increases in monthly premiums for millions with supplemental “Medigap” plans. As an infusion drug that would be administered in a doctor’s office, Aduhelm is covered by Medicare’s outpatient care benefit. The Part B standard premium, paid by most enrollees, is currently $148.50 a month.

Beyond monthly premiums, there would also be impacts on out-of-pocket costs. Many patients taking the medication, including those signed up in Medicare Advantage plans from private insurers, could face thousands of dollars in copayments,. The maximum could reach about $11,500, researchers estimated, well beyond the budget of a typical Medicare enrollee.

“Because Aduhelm is not a cure for Alzheimer’s disease, patients could incur these annual out-of-pocket costs over multiple years,” the report noted.