Earlier on Thursday, Left's Citron Research called Express Scripts the "Philidor of the pharma industry," referring to the specialty pharmaceutical company and its alleged misconduct with Valeant Pharmaceuticals. In an interview published Wednesday, Trump told Time magazine that he wants to bring down drug prices and "doesn't like what has happened" with them.
Left explained all he was saying was, "Hey, Donald Trump, if you're worried about taking drug pricing all the way down and you want some low-hanging fruit, the easiest way to start is just look at Express Scripts. Very simple."
The president-elect threatened to cancel the order of a new Air Force One after learning the cost of Boeing's new 747 was more than $4 billion. After the incident with Boeing, Left said that one can only imagine what Trump would say if he learned how much Express Scripts was profiting off of its deals.
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 puts medicine within reach by driving down cost of care and improving health outcomes.
The short-seller alleged that the lack of transparency in drug pricing benefits companies like Express Scripts. Left said, however, that he does not think this company is a "zero" or a "fraud."
"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.
Shares of Express Scripts closed at $70.75 a share on Thursday.
Express Scripts would still be profitable and valuable at that price, but "there's no reason why Express Scripts should be making more money on a drug than the actual person who manufactures the drug," Left said.
In March, the pharmacy benefits manager's largest customer, Anthem, sued Express Scripts. In the lawsuit, Anthem alleged Express Scripts was receiving "an obscene profit windfall" by pushing it to pay "massively excessive prices," The Wall Street Journal reported. At the time, a spokeswoman for Express Scripts told the Journal that the lawsuit is without merit and that it "has consistently acted in good faith and in accordance with the terms of its agreement with Anthem."
— CNBC's Bertha Coombs contributed to this report.