Allergen is the maker of Botox.
Read More I did not front-run Allergan stock, Ackman says
Ackman owns nearly a 10 percent stake in Allergan. The May 19 letter from Pershing Square Capital Management head was disclosed in a regulatory filing with the U.S. Securities and Exchange Commission. It is the latest in a series of public tactics Ackman has used to try to gain shareholder support for the deal.
"[A]s a co-bidder with Valeant Pharmaceuticals, we believe that Mr. Ackman's views and interests are not aligned with those of other Allergan stockholders," an Allergan company spokesperson said. "We note that Mr. Ackman's rhetoric focuses on, among other things, the benefits he sees in a transaction with Valeant. Nothing has changed since May 12 when, after a comprehensive review, conducted in consultation with its financial and legal advisors, the Allergan board unanimously concluded that Valeant's proposal substantially undervalued Allergan, created significant risks and uncertainties for the stockholders of Allergan, and was not in the best interests of the company and its stockholders."