Cigna called Carl Icahn's opposition to its proposed merger with Express Scripts "misguided and short-sighted" in a statement released after the market close Tuesday.

Cigna defended the deal, saying it would "create tremendous value for shareholders, accelerate the transformation of healthcare and address the dynamic regulatory environment for health services."

"Mr. Icahn, on the other hand, has made a speculative financial bet against the transaction in the hopes that he can create a gain at the expense of Cigna and Express Scripts shareholders. Mr. Icahn's opposition is misguided and short-sighted," Cigna said in a statement.

Earlier, the billionaire investor urged Cigna shareholders to vote against the proposed merger with the pharmacy benefit manager. Icahn, who has a 0.56 percent stake in Cigna, argued that "Cigna is dramatically overpaying for a highly challenged Express Scripts that is facing existential risks on several fronts."