In separate statements, Pfizer and Allergan said they have terminated their merger agreement by "mutual agreement of the companies".
The development comes after the US Treasury on Monday made 'tax inversion deals' financially less appealing. Under the 'tax inversion deals' companies could relocate their headquarters to countries with a lower tax rate.
"The decision was driven by the actions announced by the US Department of Treasury on April 4, 2016, which the companies concluded qualified as an 'Adverse Tax Law Change' under the merger agreement," Pfizer said in a statement.
Commenting on the development, Allergan CEO and President Brent Saunders said: "While we are disappointed that the Pfizer transaction will no longer move forward, Allergan is poised to deliver strong, sustainable growth built on a set of powerful attributes."
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When it was announced last year, the deal was dubbed as the biggest buyout in the healthcare sector. As per the agreement then, Allergan shareholders were to receive 11.3 shares of the combined company for each of their Allergan shares, and Pfizer stockholders were to get one share of the combined company for each of their Pfizer shares.