Biovail (BVF) sank 21% after the pharmaceutical company said a court ruled against it in a patent case involving a key drug.

The Toronto-based company said U.S. District Judge James V. Selna granted Anchen Pharmaceutical's motion for summary judgment in the Wellbutrin XL patent-infringement case.

Biovail pledged to appeal and said it would update earnings guidance next week. Wellbutrin made up about a third of the company's sales last year.

Shares fell $4.40 to $16.55.