Following a star-studded oral argument, the U.S. Court of Appeals for the Federal Circuit has lopped 15 months off of Teva Pharmaceutical Industries Ltd.’s period of market exclusivity for its blockbuster multiple sclerosis drug, Copaxone.

In a 3-0 ruling issued on Friday, the court mostly upheld an earlier ruling that generic drug applications submitted by Mylan Inc. and Sandoz AG infringe on Teva’s patents for Copaxone. The court invalidated a key Teva patent, however, allowing Mylan and Sandoz to launch generic versions of Copaxone in May 2014, more than a year earlier than many analysts expected.

Appellate heavyweight Deanne Maynard of Morrison & Foerster argued for Sandoz and its partner Momenta Pharmaceuticals Inc. Cravath, Swaine & Moore chairman Evan Chesler made the case for Mylan. Kenyon & Kenyon’s Elizabeth Holland argued at the Federal Circuit for Teva.

Teva is the world’s largest generic drug maker, but it also offers a few brand-name drugs of its own. Copaxone, the world’s best-selling drug for treating multiple sclerosis, accounts for about 20 percent of Teva’s sales and a “very significant” portion its profits, according to the company’s latest annual report. ( Reuters reported that Copoxone accounts for a whopping 50 percent of Teva’s profits.)

Some of Teva’s patents on the drug expire in 2014, while others expire in 2015. In hopes of bringing generic Copaxone to the market and gobbling up Teva’s market share, Mylan, Momenta, and Sandoz initiated Hatch-Waxman Act litigation over the drug in 2008. The cases culminated in a June 2012 ruling by U.S. District Judge Barbara Jones in Manhattan, in which she concluded that the generics would infringe nine different Teva patents. Jones also refused to invalidate the patents. The judge enjoined the generic drug makers from making the drug until September 2015, giving Teva its biggest stock jump in a decade.

Friday’s mixed decision has now given the generics reason to celebrate. While the Federal Circuit mostly affirmed Jones’s ruling, it invalidated a crucial Teva patent set to expire in September 2015. The ruling clears the way for the generics to start marketing their versions of Copaxone in May 2014, provided they can secure regulatory approval.

Kenyon’s Holland did not return a call seeking comment. MoFo’s Maynard declined to comment.