Icahn appealed to non-management board members, saying his attempts to reach an amicable agreement with Howard Solomon, Forest's chairman and chief executive, had fallen on deaf ears.

"I am concerned that we are reaching a precipice in this proxy contest from which an amicable solution is no longer possible," Icahn wrote, "and while I believe I will prevail in this proxy contest, the one thing that Howard Solomon and I do agree on, is that we both believe that an amicable solution would best serve all shareholders and would avoid a great deal of distraction and expenses for the company."

A spokeswoman for Forest, which makes the antidepressant drug Lexapro and the Alzheimer's drug Namenda, declined to comment on the letter other than to say, "We've seen it."

Icahn is Forest's second-largest shareholder, with a stake of about 10 percent. He has nominated four directors to Forest's board, persisting after having failed to win a battle for board seats last year.

Icahn said in his letter that in discussions with Solomon, he sought to avoid a proxy context by having two of his nominees replace two current board members. Solomon rejected that proposal, Icahn said.

Icahn said he then proposed that his nominees be added to the board without removing any existing members. That too was rejected, he said.

Solomon's most recent statement, Icahn said, is that the dismissal of Icahn's request came at the behest of the board.

"If you are true in your belief that a prolonged proxy contest would be deleterious to Forest (as I am), how can you be so hastily dismissive?" he wrote.

Icahn maintains that Forest is at a "critical crossroads" and that the board has allowed management to rely on two products: Lexapro, which has lost market exclusivity, and Namenda, which is set to lose it in 2015, opening the way for generic competition.

"I doubt, along with many analysts, that the current pipeline will be sufficient to pick up the slack," he said.