(Reuters) — A federal appeals court on Wednesday revived claims by several hundred plaintiffs who accused Merck & Co. of failing to adequately warn about the risks of thigh-bone fractures associated with its osteoporosis drug Fosamax.

In a 3-0 decision, the 3rd U.S. Circuit Court of Appeals in Philadelphia said the plaintiffs may proceed to trial on their failure-to-warn claims and that a lower court judge erred in finding the claims pre-empted by federal law.

Merck said it is reviewing its options and that a judge, not a jury, should decide the pre-emption question. It also said it remains "confident" in Fosamax's safety and effectiveness.

David Frederick, a lawyer for the plaintiffs, did not immediately respond to requests for comment.

Fosamax, whose chemical name is alendronate sodium, has been prescribed to treat or prevent bone loss in post-menopausal women since 1995.

But the plaintiffs claimed to suffer atypical femur fractures from long-term use, and said Merck knew about the risk for more than a decade before adding it to the Fosamax warning label in January 2011.

The Kenilworth, New Jersey-based company changed the label four months after an outside task force hired by the U.S. Food and Drug Administration published a report associating Fosamax with the fractures.

In March 2014, U.S. District Judge Joel Pisano in Trenton, New Jersey, dismissed all claims by plaintiffs injured before Sept. 14, 2010, the date of the task force report, leaving only about 20 active cases.

The judge, who retired from the bench in 2015, cited a 2009 U.S. Supreme Court decision that state law-based failure-to-warn claims were pre-empted when there was "clear evidence" that the FDA would not have approved a warning that plaintiffs requested.

Writing for the appeals court, however, Circuit Judge Julio Fuentes found enough evidence for a reasonable jury to conclude that the FDA would have approved "a properly worded warning" about Fosamax, "or at the very least, to conclude that the odds of FDA rejection were less than highly probable."

Judge Fuentes said a jury could also find that some doctors would not have prescribed Fosamax had Merck discussed the risk of fractures on a warning label.

Merck's annual sales of Fosamax regularly topped $3 billion before the company lost patent exclusivity in 2008.

Fosamax is now available as a generic. Sales of Fosamax totaled $284 million last year, down from $3.05 billion in 2007.

The case is In re: Fosamax (Alendronate Sodium) Products Liability Litigation, 3rd U.S. Circuit Court of Appeals, No. 14-1900.