Valeant's stock price has tumbled 72 percent from its high this summer with the losses accelerating in recent weeks amid questions about the company's accounting and business practices and criticisms for price gouging. It has lost 11 percent in the last five days.

Ackman's $14.5 billion Pershing Square Capital Management paid roughly $186 per share for its 21.4 million shares of Valeant. The stock closed at $72.58 on Wednesday. Last year, the hedge fund was one of the industry's best performers with a 40 percent gain.

Pershing Square has several portfolios with Pershing Square Holdings being the most aggressive, so the losses are modestly less at its other portfolios, a person familiar with the numbers.

(Reporting by Svea Herbst-Bayliss; Editing by Alan Crosby)