Lawmakers are accusing Mylan of overcharging Medicaid for the EpiPen - and the stock is tanking

Mylan could be facing another issue when it comes to the EpiPen, a drug that's gone up in price more than 500% since 2007.

The company's stock was down more than 4% Friday shortly after Sen. Ron Wyden of Oregon along with Rep. Frank Pallone of New Jersey said they sent a letter asking for more information about a possible misclassification of the EpiPen under the Medicaid program. Mylan's stock is down 17% since the drug price debate began to heat up on August 22. 

The letter, sent to Health and Human Services Secretary Sylvia Burwell, asked for more information about whether the EpiPen had been classified as a generic drug, which may have led to Mylan underpaying Medicaid. 

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Mylan did not immediately respond to a request for comment. 

Late on Thursday, Sen. Amy Klobuchar also called out the classification.

"I am deeply troubled by Mylan's misclassification of the EpiPen as a generic drug. The Minnesota Department of Human Services has estimated that this misclassification will cost our state more than $4 million dollars in overpayment this year alone. That's just one state, over the course of one year, for one drug," Klobuchar said in a statement.

Wyden also tweeted his concerns:

In the face of outrage over the price of the  EpiPen, Mylan said on Monday that it will make an "authorized generic" version of the EpiPen that will cost $300 for a two-pack. It will also continue to produce its branded version, with a list price of $600. To fend off public outrage over the EpiPen's cost - up 500% since Mylan acquired the drug in 2007 - the company had also previously raised its copay-coupon system to cover $300 of people's out-of-pocket cost.

But that discount did little to get it out of the woods.

Mylan said that it plans to launch the generic product in "several weeks," depending on when it can whip up the new labels.