Federal auditors at the Department of Health and Human Services Office of Inspector General (HHS-OIG) say errors in drugmaker-reported data stopped them from checking whether 31 Medicare Part B drugs qualified for a lower, substituted payment rate. The finding appears in a data brief dated September 21, 2026, and posted to the OIG site three days later.
The gap matters to patients as well as taxpayers. Medicare Part B generally charges enrollees 20 percent of the cost of covered drugs after the deductible, so the price Medicare recognizes for a drug sets the size of the coinsurance bill that follows.
What the inspector general’s data brief says about the 31 drugs
The brief, numbered OEI-03-26-00080 and titled “Medicare Part B Drug Payments: Impact of Price Substitutions Based on 2024 Average Sales Prices,” reports that “potential errors in the average manufacturer price data submitted to CMS prevented OIG from determining whether 31 drugs qualified for a price substitution.” In the full PDF version, the wording is that potential errors in the AMP data prevented OIG from evaluating the eligibility of 31 drugs.
The PDF adds one detail that sharpens the picture: of those 31 drugs, seven “met the 5-percent threshold in at least 1 quarter.” The brief does not say those seven would have been substituted, and it does not list the 31 drugs by name or code. Nothing in the document estimates how much money the unchecked drugs may have cost Medicare or enrollees, so any such figure would be speculation.
HHS-OIG issued the document as an institution. Neither the web page nor the PDF text names a signing official, and no addressee at the Centers for Medicare & Medicaid Services (CMS) is identified. The brief states that it “contains no recommendations,” and no CMS response appears alongside it.
How a price substitution works, and why the data comes first
Part B payment for many drugs rests on the average sales price, or ASP, which manufacturers report to CMS. A second figure, the average manufacturer price, or AMP, is also submitted to the agency. The OIG describes the safeguard this way: if it finds that a drug’s ASP exceeds its AMP by 5 percent, the law directs the HHS Secretary to replace the ASP-based payment amount with a lower rate calculated from AMPs.
That comparison is the price check named in the headline. It needs reliable numbers on both sides. When the AMP data looks wrong, the OIG cannot run the test with confidence, and the drug stays at its ASP-based payment amount by default. The brief describes the problem only as “potential” errors, which means the data was suspect rather than proven false.
What the substitution policy has delivered since 2013
The same brief gives the program’s track record. According to the OIG, CMS’s application of its price-substitution policy to drugs identified and referred by the OIG “has saved Medicare and its enrollees $78 million since 2013, including $1.6 million for 2024.” Across those years CMS has implemented substitutions for 101 drugs, 14 of them based on 2024 data.
Spread over more than a decade, and set against total Part B drug spending, those totals are modest. The 2024 figure of $1.6 million comes from only 14 drugs. The wording also credits savings to Medicare and its enrollees together, without splitting out what patients kept through lower coinsurance.
Where Part B coinsurance enters the picture
Medicare’s own cost guidance says Part B coinsurance is generally 20 percent of the cost, after the yearly deductible is met. Because coinsurance is a percentage, a lower recognized payment amount for a drug reduces the dollar coinsurance on it, and a higher one raises it. That is the sense in which enrollees share in whatever a substitution saves.
The brief does not tie the 31 drugs to any coinsurance amount, and it does not say which patients, if any, were charged more as a result of the data problem. Readers who receive Part B drugs, such as clinic-administered injections or infusions, are therefore left with a general rule rather than a measured loss: the 20 percent share applies to whatever price Medicare recognizes, and for these 31 drugs that price was not put through the OIG test.
What the record leaves open
Several questions cannot be answered from the public documents. The brief does not name the 31 drugs, does not say whether the suspected AMP errors have been corrected, and carries no CMS comment. It also does not say whether a later review will return to the seven drugs that crossed the 5-percent line in at least one quarter.
The OIG keeps its completed work in a public reports index, where follow-up evaluations or CMS actions would appear if they are published. Until then, the verified record is narrow: 31 drugs went unchecked because of questionable manufacturer data, and a policy that checks the rest has returned $78 million since 2013.
Tracking Part B drug coinsurance when a price check never happens
The OIG’s data brief shows that the price behind some Part B drugs was never tested against manufacturer pricing, while the 20 percent coinsurance still applies to whatever amount Medicare recognizes. The unfinished job for enrollees is keeping their own record of what each drug visit costs and spotting charges worth questioning.
The Medicare Cost & Coverage Protection Kit includes a medication and cost tracker, the prior-authorization appeal steps, and 51 state Medicare cost-help packs.
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This article was produced with AI assistance and checked against the primary sources linked above.
David M. Keller is a finance writer based in Columbus, Ohio, covering personal finance and consumer-focused economic topics. He earned his degree in journalism from Ohio University and began his career reporting on local business and economic trends for a regional media outlet. Since then, he has contributed to a variety of online publications, focusing on clear, practical coverage of topics such as cost of living, debt, and everyday financial decision-making.
