Amgen vows to fight after IRS demands $3.6B in back taxes—and a bigger bill could be on the way

No one enjoys owing the IRS, especially when the bill totals $3.6 billion.

But that’s apparently the amount in back taxes the U.S. tax collecting agency is seeking from Amgen, not including interest, for the years 2010, 2011 and 2012, according to the company’s earnings release on Tuesday.