Why Allergan (AGN) Stock Soared in After Market Trading Today

NEW YORK (TheStreet) -- Allergan  (AGN) soared in after-hours trading on Monday amid news that activist investor William Ackman and Valeant Pharmaceuticals International have joined forces to pursue a takeover of the health care company.

Ackman is the founder and CEO of hedge fund Pershing Square Capital Management, which has built an almost 10% stake in Allergan, according to CNBC. The company is worth approximately $4 billion.

Allergan closed at $142, an all-time high, and then surged 19% after the market closed. More than 9 million shares changed hands, more than triple the average volume of 2,944,310. 

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AGN Chart

AGN data by YCharts

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