Over the nine or so years that Mylan, Inc. has been selling—and hiking the price—of EpiPens, the drug company has been misclassifying the life-saving device and stiffing Medicaid out of full rebate payments, federal regulators told Ars.

Under the Medicaid Drug Rebate Program, drug manufacturers, such as Mylan, can get their products covered by Medicaid if they agree to offer rebates to the government to offset costs. With a brand-name drug such as the EpiPen, which currently has no generic versions and has patent protection, Mylan was supposed to classify the drug as a “single source,” or brand name drug. That would mean Mylan is required to offer Medicaid a rebate of 23.1 percent of the costs, plus an “inflation rebate” any time Mylan raises the price of the brand-name drug at a rate higher than inflation.

Mylan has opted for such price increases—a lot. Since Mylan bought the rights to EpiPen in 2007, it has raised the price on 15 separate occasions, bringing the current list price to $608 for a two-pack up from about $50 a pen in 2007. That’s an increase of more than 500 percent, which easily beats inflation.

But instead of classifying EpiPen as a “single source” drug, Mylan told regulators that it’s a “non-innovator multiple source,” or generic drug. Under that classification, Mylan is only required to offer a rebate of 13 percent and no inflation rebates.

It’s unclear how much money Mylan has skipped out on paying in total to state and federal governments. But according to the state health department of Minnesota, as reported by CNBC, the misclassification cost that state $4.3 million this year alone.

In a statement to Ars, Aaron Albright, a spokesperson for the Centers for Medicare and Medicaid Services (CMS), said that regulators had told Mylan of the mistake “on multiple occasions”:

CMS has… expressly advised Mylan that their classification of EpiPen for purposes of the Medicaid Drug Rebate program was incorrect. This incorrect classification has financial consequences for the amount that federal and state governments spend by reducing the amount of quarterly rebates Mylan owes for EpiPen.

Those financial consequences may include penalties and other government claims. However, Albright declined to discuss potential penalties they’re pursuing, saying only: “We are not in a position to comment on specific steps taken by CMS or our other federal partners to correct this misclassification.”

The question of whether Mylan had misclassified EpiPens came up during a recent Congressional hearing of the House Oversight and Government Reform Committee. Mylan CEO Heather Bresch, defending the company’s prices at the hearing, stood by the classification. She noted that EpiPen was classified as a generic before Mylan bought the drug in 2007.