Inversions are back, whether Congress is prepared or not.

Pfizer, one of the world's largest pharmaceutical companies, announced Thursday that it was in "preliminary friendly discussions" with pharmaceutical company Allergan about combining to form the world's largest drug company.

If the deal goes through, the headquarters of the combined company could be placed in Dublin, home to Allergan, rather than New York, where Pfizer is headquartered. That move would allow the newly formed business to take advantage of Ireland's lower corporate tax rates.

The threat of Pfizer moving its headquarters outside the U.S. would realize the fears of lawmakers such as newly installed House Speaker Paul Ryan, who has sought to reform the corporate tax code. It also has drawn the ire of Democratic lawmakers who view such tax maneuvers as corporate disloyalty.

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