Biogen Inc.’s new $56,000-a-year Alzheimer’s drug could wind up costing thousands of dollars more per patient than the company has said because of quirks in the way the therapy and others like it are packaged and paid for.

Medicare and other insurers could wind up paying $61,000 to $62,000 a patient on average depending on the dosages each needs, according to an analysis published online Friday by the journal Health Affairs.

The discrepancy stems from the way that infused drugs like the Alzheimer’s drug, named Aduhelm, are administered: based on a patient’s body weight.

The heavier a patient is, the bigger dose they need, while lighter patients require lower doses. Drugmakers package and price the infused therapies by the vial, which means an infused therapy costs more if a person’s weight requires an additional vial.

In the case of Aduhelm, patients receive 10 milligrams for every kilogram of body weight, so that lighter patients use a lower dose than heavier ones.

Biogen based Aduhelm’s $56,000 annual list price on the three vials required to treat a patient weighing 74 kilograms, or 163 pounds. The company says that is the average weight of a U.S. patient with mild Alzheimer’s symptoms that the drug was designed to treat.

For patients weighing 170 pounds, the annual price would near $59,000 annually, according to the analysis, which was done by Peter Bach, director of the Drug Pricing Lab at Memorial Sloan Kettering Cancer Center and Chief Medical Officer of Delfi Diagnostics.

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At 180 pounds, the annual price tag goes up more than 10% to $61,880 annually. At 200 pounds, the price is about $68,430, or 22% more than Biogen’s price estimate, Dr. Bach said.

For patients weighing less than about 150 pounds, the price drops below Biogen’s estimate of $56,000 annually. A patient weighing 149 pounds, for instance, would require a dose costing about $49,500 annually.

Doctors would need to use more vials for heavier patients, raising Aduhelm’s cost. And regardless of a patient’s body weight, doctors are likely to have to discard excess drug left over in the vial, though health insurers will have to pay for the whole vial.

Doctors might discard any excess drug left over in the vial, though health insurance would have to pay for the whole vial.

A Biogen spokesman said Dr. Bach’s analysis was flawed because it doesn’t specifically analyze the body weights of Alzheimer’s patients.

Dr. Bach’s analysis, the spokesman said, overestimated the amount of unused drug that doctors would discard. “Biogen works with physicians to ensure efficient use of vials to minimize wastage,” the spokesman said.

Doctors are likely to have to discard excess Aduhelm left over in the vial, though health insurers will have to pay for the whole vial. A Biogen lab.

Doctors are likely to have to discard excess Aduhelm left over in the vial, though health insurers will have to pay for the whole vial. A Biogen lab.

Photo: David A. White/Biogen/Handout/EPA-EFE/Shutterstock

The U.S. Food and Drug Administration last month approved Aduhelm, the first green light for an Alzheimer’s treatment in nearly two decades.

Patients and caregivers hailed the decision as adding a desperately needed treatment to the limited medicine chest for Alzheimer’s, a progressive disease affecting about six million people in the U.S.

Some FDA staffers and advisers opposed the drug’s approval, saying there wasn’t sufficient evidence to show the drug works.

Some doctors and researchers also criticized the drug’s list price. Health economists warned that Aduhelm’s cost could quickly add billions of dollars in new spending by Medicare, the federal insurance program for the elderly and disabled that is expected to pay for the majority of prescriptions.

The FDA said Friday it would ask federal health officials to investigate communications between agency staff and Biogen before Aduhelm’s approval.

For the analysis, Dr. Bach said he used data on the average body weights of men and women in the U.S. to calculate the average price Biogen will receive for its drug.

The average price, he said, will be between $61,000 and $62,000 annually depending on the percentage of women and men treated with the drug.

Aduhelm is packaged in two vial sizes: 300 milligrams and 170 milligrams.

Dr. Bach estimated around 30 milligrams of drug would be leftover and likely discarded after each monthly dose for each 163-pound patient that Biogen said would be typical. Over a year, the discarded drug would add up to an additional $2,184 annually paid by insurers, according to Dr. Bach, who provided the estimate to The Wall Street Journal.

The patient weighing 180 pounds would require additional vials of drug to meet their dosage level, at a total list price of $61,880 annually, including about $2,400 in discarded drug costs, according to Dr. Bach.

If 500,000 patients were to receive full doses over the course of a year, excess discarded drug would result in an additional $2 billion in annual revenue for Biogen, Dr. Bach estimated.

Aduhelm is an example of a long-running flaw in the payment system for infused and injected drugs administered in doctor’s offices, Dr. Bach said. Many cancer and vision-loss drugs are administered via IV or injections rather than swallowed in a pill.

For  Medicare patients, doctors and hospitals buy the drugs directly from wholesalers and then bill the government for reimbursement as well as an additional payment based on a percentage of the drug’s price to cover overhead costs.

Drugmakers typically package their drugs in a limited number of vial sizes, which leads to leftover medication because each patient’s dose often don’t exactly match the vial size. The leftover drug substance is often discarded by the doctor’s office, but Medicare pays for all of it.

In 2016, Dr. Bach and colleagues estimated that the U.S. healthcare system spent $2 billion annually because of discarded drug from cancer-drug vials that were larger than they needed to be.

Variability in how much patients weigh, Dr. Bach said, allows companies to publicize list prices underestimating the true costs.

“It’s unfair play that leads to a huge boost in profit,” says Dr. Bach. “They’re taking advantage of the fact that the claims and billing system is such a morass and they are able to sneak in extra revenue that they are not open about.”

GlaxoSmithKline PLC said in 2020 that the price for its multiple-myeloma drug Blenrep would be about $23,900 a month per patient based on the average patient weight of 175 pounds, Dr. Bach said.

Some 44% of the U.S. adults are heavier than that, Dr. Bach said, indicating that the monthly cost will be $30,480. The drug’s prescribing instructions, meanwhile, instruct pharmacists to discard any unused drug, he says.

A Glaxo spokeswoman said the company packages Blenrep in way that minimizes waste by filing vials with a dosage strength based on the average baseline weight of the majority of patients in its clinical trials.

In 2019,  the Senate Finance Committee’s highest-ranking members Sen. Chuck Grassley (R., Iowa) and ranking member Ron Wyden (D., Ore.) introduced sweeping legislation to address high drug-prices that included a requirement that drugmakers refund Medicare for any payments received for discarded drug. The Congressional Budget Office projected the provision would save $9 billion over 10 years. The bill gained some bipartisan support but died as the 2020 campaign season ramped up.

Write to Joseph Walker at joseph.walker@wsj.com