(Adds background on case, quotes from justices)
By Andrew Chung
WASHINGTON, Jan 7 (Reuters) - U.S. Supreme Court justices on Monday appeared sympathetic to Merck & Co in its bid to fend off hundreds of lawsuits accusing the company of failing to adequately warn patients of the risks of thigh bone fractures associated with its osteoporosis drug Fosamax.
The justices heard arguments in Merck's appeal of a lower court ruling that revived the lawsuits filed in federal court by patients who said they suffered debilitating bone breaks after taking the drug.
Some conservative justices as well as liberal Justice Stephen Breyer emphasized that the U.S. Food and Drug Administration had been uncertain about the fracture risk and rejected Merck's proposal to add a warning label to the medication.
The nine-justice court, which has a 5-4 conservative majority, must decide whether a pharmaceutical company can be held liable under such circumstances.
A ruling for Merck, which is backed by President Donald Trump's administration in the case, would be a relief for drug companies as it would preserve one of their most powerful defenses against product liability claims by contending they are preempted by the actions of a federal agency under both federal law and the U.S. Constitution.
Fosamax helps prevent and treat osteoporosis, a condition that can lead to bone fractures, in women who have gone through menopause. It may increase in the risk of fractures in the thigh bone or just below the hip joint, often requiring surgical intervention.
Sales of Fosamax, which is also available as a generic drug, totaled $241 million in 2017, according to Merck.
Merck in 2008 submitted data to the FDA suggesting Fosamax might be linked to certain bone fractures, but the FDA denied its proposal to add a warning label to the medication. After a task force further studied the issue, the agency in October 2010 ordered manufacturers to revise labels to include a warning, which Merck did.
The plaintiffs said that the FDA rejected only Merck's proposed language for the warning that focused on stress fractures rather than the more serious fractures they suffered.
Breyer, along with conservative justices Samuel Alito, Neil Gorsuch and Chief Justice John Roberts, appeared skeptical of the plaintiffs' arguments and stressed the need to be cautious about warnings on drugs. Over-warning, Breyer suggested, can hurt the majority of patients "who can benefit from this medicine."
Liberal Justices Elena Kagan and Sonia Sotomayor said Merck had a duty to present proper data and relevant warning labels to the FDA instead of letting the agency figure out what to do.
"Seems to be sort of turning responsibility on its head," Sotomayor said.
Fosamax users sued New Jersey-based Merck in federal courts alleging the drug caused them to suffer serious thigh bone fractures and that the company failed to warn of the risk. The number of cases has swelled to more than 1,000.
A federal trial court in New Jersey threw out the cases, but in 2017 the Philadelphia-based 3rd U.S. Circuit Court of Appeals allowed the claims to proceed to trial, saying that a jury could find that the FDA had objected only to Merck's phrasing of the proposed warning label.
Liberal Justice Ruth Bader Ginsburg did not participate in the oral argument as she continues to recuperate after lung cancer surgery, but will take part in the ruling.
Reporting by Andrew Chung; Editing by Will Dunham