Shares of Express Scripts dipped more than 2 percent Friday after Citron Research called the company the "Gotti of Pharma."
The short-selling firm said on Twitter that new data from Centers for Medicare & Medicaid Services is "indisputable" and that "rebates damage" the system.
In a statement to CNBC, Express Scripts pushed back against the firm's comments and said it is a "market force that makes medicine affordable."
"Rebates don't raise drug prices, drug makers do," the company said in a statement to CNBC.
Citron's tweet referred to John Gotti, an American gangster who became boss of the Gambino crime family in New York City.
Citron also posted a note on their website, saying "One company in the healthcare system that deserves to fall to the mighty sword of the new Trump administration is Express Scripts."
This isn't the first time Citron has criticized the pharmacy benefit management company. Late last year, short-seller Andrew Left, author and editor of the online investment newsletter,said if Trump wants to tackle drug pricing, he should look toward low-hanging fruit like Express Scripts.
"What I'm saying is that they have to get normalized margins and normalized profits along with everyone else in that whole channel of drugs ... Once that is done, this stock could easily contract to from where it is right now to $40 or $45 a share," Left said in December.
With Friday's declines, the stock has shed more than 12 percent in the past 6 months.
—CNBC's Christine Wang contributed to this report.