American depositary receipts of the Dublin company at last check were off 3.2% at $4.88.
The U.S. Court of Appeals for the Federal Circuit in Washington upheld the March ruling by a U.S. District Court in Nevada that invalidated six patents on the drug and wiped out $3.5 billion in market value.
Generic-drug makers Hikma Pharmaceuticals HKMPY and Dr. Reddy's Laboratories (RDY) - Get Free Report had sought to overturn the patents so they could begin to produce generic versions of the medication, which is based partly on fish oil.
The appeals court made its ruling without opinion one day after hearing arguments in the case.
Stifel analyst Derek Archila had said Wednesday in a note to clients that the hearing concluded after "all of 20 minutes."
"We are extremely disappointed with today’s ruling and plan to vigorously pursue available remedies,” John Thero, president and chief executive of Amarin, said in a statement.
“Importantly, we and our partners are continuing to pursue additional regulatory approvals for Vascepa in China, Europe and the additional countries in the Middle East, and remain confident in the global market potential of Vascepa."
Thero said the company was preparing for an expected approval of Vascepa with a launch early next year. Amarin will continue to meet the demand in the U.S., he said, and continue current promotion levels of Vascepa there.
"After assessing the scope, timing and pricing of potential generic competition, Amarin will decide whether to further expand, contract or maintain such levels of Vascepa promotion," Thero said.
Countries outside the U.S. where Vascepa is sold and is under regulatory review are not subject to this litigation and judgment, he added, and no generic litigation is pending outside the U.S.
Hikma received FDA clearance for the product in May 2020 and is working toward a launch.
"We are very pleased with the Federal Circuit's swift decision and to be one step closer to launching a generic version of this important medicine for US patients and healthcare providers, helping us to continue putting better health within reach every day," Hikma President Brian Hoffmann said in a statement.