Johnson & Johnson said a U.S. court had held its patent for autoimmune drug Remicade invalid, ruling in favor of two companies seeking to bring a cheaper version of the blockbuster drug to market.
Remicade, J&J's biggest product, had $6.6 billion in sales last year. The drug's patent will lapse in September 2018.
In March 2015, J&J filed a lawsuit against Celltrion Inc and Pfizer Inc-owned Hospira seeking a declaratory judgment that their biosimilar product infringes several of its patents.
Celltrion and Hospira's biosimilar, Inflectra, received U.S. regulatory approval in April.
Biosimilars are lower-cost copies of complex biotech drugs.
J&J, which is also defending other Remicade patents, said it plans to appeal the latest court decision.
"Assuming biosimilar competition, we think Remicade revenues will decline by just over $1 billion in 2017, costing J&J close to $0.20 in EPS," J.P. Morgan analysts wrote in a client note.
(Reporting by Shailesh Kuber in Bengaluru; Editing by Maju Samuel)