Amgen Shares Fall Most In a Year as IRS Seeks $7.1 Billion

Shares of Amgen Inc. fell the most in a year after the company disclosed that it is facing a potential tax bill of at least $7.1 billion from the U.S. Internal Revenue Service.

The IRS is seeking $5.1 billion in back taxes related to how profit was allocated between Amgen’s entities in Puerto Rico and in the mainland U.S. for the years 2013 to 2015, in addition to about $2 billion in penalties. The drugmaker disclosed the news with its first-quarter results on Wednesday, and it largely overshadowed better-than-expected revenue and earnings.

Many pharmaceutical companies have operations in Puerto Rico, ...