WASHINGTON - Chief Justice John G. Roberts Jr. declined Friday to temporarily block a lower-court ruling that opens the world's best-selling multiple-sclerosis drug to competition from generic rivals next month.

The decision is a victory for rivals challenging the patents of Israeli-based Teva Pharmaceutical Industries Ltd., maker of Copaxone. Teva's Americas headquarters is in North Wales, Montgomery County.

Teva claims the U.S. Court of Appeals for the Federal Circuit wrongly overturned five of its patents for the drug. That ruling allows rivals Mylan Inc., Momenta Pharmaceuticals Inc., and Sandoz Inc. to start selling cheaper generic versions in May instead of September 2015.

The Supreme Court has agreed to consider the case, but arguments will not take place until its new term begins in October and it could be next year before a decision is reached. Teva said it would suffer irreparable harm if the appeals court decision was not postponed. Copaxone brought the company $3.2 billion in U.S. sales last year.

In a one-page ruling, Roberts said he was not convinced Teva would suffer such harm. If Teva prevails in the case, Roberts said, the company would be able to recover damages from the generic rivals for patent infringement. He acknowledged that Teva has "a fair prospect" of winning the case at the high court.

Roberts oversees emergency appeals from the U.S. Court of Appeals for the Federal Circuit, which hears appeals in patent cases.

It is the second time Roberts has declined to put the appeals court ruling on hold. He rejected a similar request from Teva in November while the high court was deciding whether to take up the appeal.

The rival companies had argued that granting a stay would effectively extend Teva's monopoly for years to come. That's because Teva is trying to switch existing Copaxone patients to a new formulation of the drug that has patent protection until 2030.