An FDA approval is required before a new animal drug can be introduced into interstate commerce. The FDA approval occurs in two phases: a testing phase, followed by an approval phase. If there is a patent on the new animal drug, the patent owner may request for extension of the patent term to restore the patent life lost during the duration of the FDA review. The extension that may be obtained is half of the duration of the test phase plus the entire duration of the approval phase. The extension cannot exceed five years and cannot include any regulatory review period occurring before the patent issues.

In Wyeth and Elan Pharma International, Ltd. v. Under Sec. of Commerce for Intellectual Property (Federal Circuit 2009-1120, 7 January 2010), the Federal Circuit visited the question of how to restrict the period of adjustment under 35 USC 154(b)(1) when any of the periods of delay overlap.