March's poor performance leaves Ackman's fund with a 25.6 percent loss for the year, a person familiar with its returns said on Friday. This marks a dramatic turnaround from Pershing Square's 40 percent gain in 2014.

Pershing Square owns a 9 percent stake in Valeant and the company's stock tumbled last month when it cut its financial forecasts and said it could face default if it fails to file its annual report by the end of April. Last week the company's chief executive officer agreed to step down and Ackman formally joined the board.

(Reporting by Svea Herbst-Bayliss; Editing by Chris Reese)