The US Supreme Court vacated and remanded a products liability case involving claims alleging a failure to warn about risks of stress fractures related to the use of Fosamax, which is manufactured by Merck Sharpe & Dohme Corp. (“Merck”).

Fosamax is a drug that treats and prevents osteoporosis in post-menopausal women. Merck argued that the claims should be dismissed as they submitted “clear evidence” that FDA regulations prohibited them from changing the Fosamax label to include the stress fracture risk warnings. The Supreme Court held that the Third Circuit incorrectly determined that “clear evidence” is a question of fact for a jury; rather, the Court held that “clear evidence” is a question of law as it involves a legal determination about an agency decision.

The FDA approved the sale of Fosamax in 1995. At that time, the Fosamax label did not warn of the risk of atypical femoral fracture, a type of stress fracture in the thigh bone. However, through post-market surveillance, Merck later concluded that the use of Fosamax was associated with an increase risk of these fractures. In 2008, Merck submitted to the FDA a labeling change proposal that would have included the risk of “stress fractures,” however the FDA rejected that proposal. Merck contends that FDA would also have rejected a labeling change proposal with the more specific language warning of the risk of “atypical stress fracture.”

500 individuals who took Fosamax between 1999 and 2010 and suffered atypical femoral fractures sued Merck under state tort laws with the theory that state laws imposed on Merck a duty to warn of the risk of these atypical femoral fractures. In the District Court, Merck argued that the respondents’ state law claims should be dismissed as pre-empted by federal law, as it would have been impossible for Merck to comply with both state law (which requires risk warnings) and federal law (which prohibits labeling changes unless approved by the FDA). The District Court agreed with Merck, and granted summary judgment dismissing the suit.

The U.S. Supreme Court on Monday handed a victory to Merck & Co – at least for now – by throwing out a lower court ruling that had revived hundreds of lawsuits accusing the company of failing to properly warn patients of debilitating thigh-bone fractures from taking its osteoporosis drug Fosamax.

The nine justices unanimously directed the Philadelphia-based 3rd U.S. Circuit Court of Appeals to reconsider its decision allowing the lawsuits to proceed even though federal regulators had rebuffed Merck when the company sought to add a warning to Fosamax’s label about the fracture risk.

The decision added clarity to a defense used by drug makers against product liability claims in which they argue that such litigation is preempted under federal law and the U.S. Constitution when a federal agency takes certain actions, as the Food and Drug Administration did in this case.

In a decision written by Justice Stephen Breyer, the court said that judges, not juries, must decide disputes over preemption, and that a drug manufacturer must show that it “fully informed” the FDA of the need for a warning before it was rebuffed. Lower courts now must revisit the case reflecting the Supreme Court’s guidance.