Bill Ackman isn't the only hedge fund manager who likely made big money on the Allergan deal.
Actavis announced on Monday that it will acquire Allergan for $66 billion, or $219 per share in cash and Actavis shares. The stock had been trading bear $110 earlier this year.
Allergan is one of the most widely held stocks by hedge funds. As of Sept. 30, some 19 percent of hedge funds owned it, according to an analysis of public holdings by data analysis company Symmetric.
The largest holder is Ackman's Pershing Square Capital Management, with $5.14 billion worth of shares (28.8 million) as of Sept. 30. The firm bought the stock at an average price of $127; at the current price, that means a more than $2 billion gain after a profit-sharing arrangement with Valeant. (Ackman had tried to buy Allergan by teaming up with Valeant earlier this year.)