Pershing Square lost $4.1B on Valeant — Bill Ackman's worst investment ever

Pershing Square Capital Management, Bill Ackman's hedge fund, built a massive position in Valeant Pharmaceuticals beginning in 2015, eventually holding roughly 27 million shares worth billions. Ackman joined Valeant's board and became its public champion, defending the company against critics.

Valeant's business model was built on acquiring undervalued drugs and sharply raising their prices — a strategy that generated huge revenue growth but attracted intense scrutiny. In 2015, short-seller Andrew Left's Citron Research published reports accusing Valeant of fraud, including using a mail-order pharmacy (Philidor) to manufacture fake sales and conceal revenue problems.

The stock collapsed from $263 per share in August 2015 to around $10 by 2017. Pershing Square's position lost approximately $4.1 billion — one of the largest single-stock losses in hedge fund history. Ackman called it his 'worst investment ever' and left Valeant's board in 2017, eventually selling his remaining stake at a massive loss.

The loss also devastated Pershing Square's performance: the fund lost 20.5% in 2015 and 13.5% in 2016, its worst years ever, and investors redeemed billions. Ackman's reputation as a 'genius investor' was permanently damaged, and the fund never fully recovered its peak size.