Express Scripts shares fell as much as 10 percent after Citron Research tweeted that the pharmacy benefit management company is the "culprit" behind pharmaceutical price gouging.
It later recovered some of those losses, but still ended the day more than 6 percent lower.
Citron Research, the short-selling research firm run by Andrew Left, tweeted that Express Scripts is the "Philidor of the pharma industry," referring to Philidor Rx Services' alleged misconduct with Valeant Pharmaceuticals.
Express Scripts told CNBC that without having specific knowledge of the report, it can only discuss in general terms. It said Express Scripts is a market force that uniquely puts medicine within reach by driving down cost of care and improving health outcomes.
With Wednesday's declines, Express Scripts shares have fallen 19 percent this year.
Andrew Left, executive editor of Citron Research, will be on CNBC's "Fast Money" Thursday at 5 p.m. ET.