Merck Sharp & Dohme Corp. (defendant) manufactures Fosamax, a drug used to treat and prevent osteoporosis. In 1998, the Food and Drug Administration (FDA) approved Fosamax labelling without a warning about unusual thigh-bone stress fractures, although Merck scientists knew Fosamax could theoretically cause them. Merck applied to add a warning in 2008, but the FDA rejected the application. At the time, neither Merck nor the FDA was certain that the developing evidence showed a connection strong enough to require a new warning. The FDA did not require Merck to add a warning until 2010. Meanwhile, from 1999 to 2010, more than 500 women (plaintiffs) who took Fosamax suffered stress fractures and brought failure-to-warn lawsuits against Merck. Merck countered that federal law precluded state-law failure-to-warn claims because the FDA would not and did not approve an additional warning in 2008. The trial court agreed and granted summary judgment for Merck. The appellate court reversed and remanded, reasoning that the jury should decide whether it was highly unlikely that the FDA would have approved the label change. Merck petitioned the Supreme Court for review.