Diana Levine (plaintiff) was treated for a migraine headache with an injection of Phenergan, a drug manufactured by Wyeth (defendant). Phenergan is very dangerous and can cause gangrene if injected into an individual’s artery. When the physician’s assistant injected Levine, the drug somehow got into Levine’s artery, necessitating amputation of her forearm. Levine brought common-law negligence and strict-liability claims against Wyeth, asserting that the warning label on the drug was inadequate. The U.S. Food and Drug Administration (FDA) had deemed the Phenergan label sufficient prior to the drug’s approval; however, the FDA had also implemented a regulation stating that a manufacturer is entitled to add to or strengthen a warning on a label without prior FDA approval. The trial court ruled in Levine’s favor, finding based on state law that Wyeth failed to properly warn of the dangers of Phenergan. The Vermont Supreme Court affirmed. Wyeth appealed to the United States Supreme Court, arguing that Levine’s claims based on state law were preempted by federal law.